-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 S2YyROP9WNR3mWPiAwVBdkhbLFs6VT/S7UYmP7RUuiWER4tzuir6Ll2i1OIcQFp1
 CxcvUGbqd1YH9NFKEbYNRg==

<SEC-DOCUMENT>0001123292-09-000111.txt : 20090205
<SEC-HEADER>0001123292-09-000111.hdr.sgml : 20090205
<ACCEPTANCE-DATETIME>20090205152500
ACCESSION NUMBER:		0001123292-09-000111
CONFORMED SUBMISSION TYPE:	3
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090127
FILED AS OF DATE:		20090205
DATE AS OF CHANGE:		20090205

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ARCA biopharma, Inc.
		CENTRAL INDEX KEY:			0000907654
		STANDARD INDUSTRIAL CLASSIFICATION:	IN VITRO & IN VIVO DIAGNOSTIC SUBSTANCES [2835]
		IRS NUMBER:				363855489
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021
		BUSINESS PHONE:		720-940-2200

	MAIL ADDRESS:	
		STREET 1:		8001 ARISTA PLACE
		STREET 2:		SUITE 200
		CITY:			BROOMFIELD
		STATE:			CO
		ZIP:			80021

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	NUVELO INC
		DATE OF NAME CHANGE:	20030203

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	HYSEQ INC
		DATE OF NAME CHANGE:	19970610

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Atlas Venture Fund VII L P
		CENTRAL INDEX KEY:			0001346450
		STATE OF INCORPORATION:			DE

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-22873
		FILM NUMBER:		09572872

	BUSINESS ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		7816221700

	MAIL ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Atlas Venture Associates VII, L.P.
		CENTRAL INDEX KEY:			0001377726
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-22873
		FILM NUMBER:		09572871

	BUSINESS ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		781 622 1700

	MAIL ADDRESS:	
		STREET 1:		890 WINTER STREET
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
</SEC-HEADER>
<DOCUMENT>
<TYPE>3
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2009-01-27</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000907654</issuerCik>
        <issuerName>ARCA biopharma, Inc.</issuerName>
        <issuerTradingSymbol>ABIO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001346450</rptOwnerCik>
            <rptOwnerName>Atlas Venture Fund VII L P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1></rptOwnerStreet1>
            <rptOwnerStreet2>890 WINTER STREET, SUITE 320</rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001377726</rptOwnerCik>
            <rptOwnerName>Atlas Venture Associates VII, L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>890 WINTER STREET, SUITE 320</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1402163</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Warrant to Purchase Common Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>9.7406</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2013-10-10</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>75449</value>
                    <footnoteId id="F3"/>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">These shares are held directly by Atlas Venture Fund VII, L.P.  Atlas Venture Associates VII, L.P. is the general partner of Atlas Venture Fund VII, L.P.  Each of the Filing Persons disclaims beneficial ownership of the shares except to the extent of his or its pecuniary interest therein.</footnote>
        <footnote id="F2">This warrant is immediately exercisable.</footnote>
        <footnote id="F3">These warrants are held directly by Atlas Venture Fund VII, L.P.  Atlas Venture Associates VII, L.P. is the general partner of Atlas Venture Fund VII, L.P.  Each of the Filing Persons disclaims beneficial ownership of the warrants except to the extent of his or its pecuniary interest therein.</footnote>
    </footnotes>

    <remarks>This statement on Form 3 is filed by Atlas Venture Fund VII, L.P. and Atlas Venture Associates VII, L.P.  The principal business address for each of the reporting persons is 890 Winter Street, Suite 320, Waltham, Massachusetts 02451.  The reporting persons disclaim beneficial ownership of the securities listed herein except to the extent of their pecuniary interest therein.</remarks>

    <ownerSignature>
        <signatureName>Atlas Venture Fund VII, L.P.
By:  Atlas Venture Associates VII, L.P.
Its General Partner
By:  Atlas Venture Associates VII, Inc.
Its General Partner

By:/s/ Kristen Laguerre
Name:  Kristen Laguerre
Title:  Vice President</signatureName>
        <signatureDate>2009-02-05</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>Atlas Venture Fund VII, L.P.
By:  Atlas Venture Associates VII, Inc.
Its General Partner

By: /s/ Kristen Laguerre
Name:  Kristen Laguerre
Title:  Vice President</signatureName>
        <signatureDate>2009-02-05</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----
