-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 PXyQrJIqrBqRVVHtOKhgcgbSoGZ6QIZRBQUCfGmvE3Eh/Tw+mcJFLSqrZoImdtRe
 ioSFNZkl+pKlCxhosJ51Lg==

<SEC-DOCUMENT>0001442836-10-000003.txt : 20100226
<SEC-HEADER>0001442836-10-000003.hdr.sgml : 20100226
<ACCEPTANCE-DATETIME>20100226102147
ACCESSION NUMBER:		0001442836-10-000003
CONFORMED SUBMISSION TYPE:	D
PUBLIC DOCUMENT COUNT:		1
ITEM INFORMATION:		Rule 506
FILED AS OF DATE:		20100226
DATE AS OF CHANGE:		20100226
EFFECTIVENESS DATE:		20100226

FILER:

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MERSANA THERAPEUTICS INC
		CENTRAL INDEX KEY:			0001442836
		IRS NUMBER:				000000000
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		D
		SEC ACT:		1933 Act
		SEC FILE NUMBER:	021-121447
		FILM NUMBER:		10636561

	BUSINESS ADDRESS:	
		STREET 1:		840 MEMORIAL DRIVE
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
		BUSINESS PHONE:		617-498-0020

	MAIL ADDRESS:	
		STREET 1:		840 MEMORIAL DRIVE
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02139
</SEC-HEADER>
<DOCUMENT>
<TYPE>D
<SEQUENCE>1
<FILENAME>primary_doc.xml
<TEXT>
<XML>
<?xml version="1.0"?>
<edgarSubmission>

    <schemaVersion>X0603</schemaVersion>

    <submissionType>D</submissionType>

    <testOrLive>LIVE</testOrLive>

    <primaryIssuer>
        <cik>0001442836</cik>
        <entityName>MERSANA THERAPEUTICS INC</entityName>
        <issuerAddress>
            <street1>840 MEMORIAL DRIVE</street1>
            <city>CAMBRIDGE</city>
            <stateOrCountry>MA</stateOrCountry>
            <zipCode>02139</zipCode>
        </issuerAddress>
        <issuerPhoneNumber>617-498-0020</issuerPhoneNumber>
        <jurisdictionOfInc>DELAWARE</jurisdictionOfInc>
        <issuerPreviousNameList>
            <value>None</value>
        </issuerPreviousNameList>
        <edgarPreviousNameList>
            <value>None</value>
        </edgarPreviousNameList>
        <entityType>Corporation</entityType>
        <yearOfInc>
            <overFiveYears>true</overFiveYears>
        </yearOfInc>
    </primaryIssuer>

    <relatedPersonsList>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Julie</firstName>
                <middleName>A.</middleName>
                <lastName>Olson</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Peter</firstName>
                <lastName>Leone</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Executive Officer</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Nicholas</firstName>
                <middleName>G.</middleName>
                <lastName>Bacopoulous</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>C.</firstName>
                <middleName>Clarke</middleName>
                <lastName>Boyd</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Joyce</firstName>
                <lastName>Tsang</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Martin</firstName>
                <lastName>Vogelbaum</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
        <relatedPersonInfo>
            <relatedPersonName>
                <firstName>Thomas</firstName>
                <lastName>Beck</lastName>
            </relatedPersonName>
            <relatedPersonAddress>
                <street1>840 Memorial Drive</street1>
                <city>Cambridge</city>
                <stateOrCountry>MA</stateOrCountry>
                <zipCode>02139</zipCode>
            </relatedPersonAddress>
            <relatedPersonRelationshipList>
                <relationship>Director</relationship>
            </relatedPersonRelationshipList>
            <relationshipClarification></relationshipClarification>
        </relatedPersonInfo>
    </relatedPersonsList>

    <offeringData>
        <industryGroup>
            <industryGroupType>Biotechnology</industryGroupType>
        </industryGroup>
        <issuerSize>
            <revenueRange>Decline to Disclose</revenueRange>
        </issuerSize>
        <federalExemptionsExclusions>
            <item>06</item>
        </federalExemptionsExclusions>
        <typeOfFiling>
            <newOrAmendment>
                <isAmendment>false</isAmendment>
            </newOrAmendment>
            <dateOfFirstSale>
                <value>2010-01-19</value>
            </dateOfFirstSale>
        </typeOfFiling>
        <durationOfOffering>
            <moreThanOneYear>false</moreThanOneYear>
        </durationOfOffering>
        <typesOfSecuritiesOffered>
            <isDebtType>true</isDebtType>
        </typesOfSecuritiesOffered>
        <businessCombinationTransaction>
            <isBusinessCombinationTransaction>false</isBusinessCombinationTransaction>
            <clarificationOfResponse></clarificationOfResponse>
        </businessCombinationTransaction>
        <minimumInvestmentAccepted>0</minimumInvestmentAccepted>
        <salesCompensationList></salesCompensationList>
        <offeringSalesAmounts>
            <totalOfferingAmount>3500000</totalOfferingAmount>
            <totalAmountSold>2500000</totalAmountSold>
            <totalRemaining>1000000</totalRemaining>
            <clarificationOfResponse></clarificationOfResponse>
        </offeringSalesAmounts>
        <investors>
            <hasNonAccreditedInvestors>false</hasNonAccreditedInvestors>
            <totalNumberAlreadyInvested>7</totalNumberAlreadyInvested>
        </investors>
        <salesCommissionsFindersFees>
            <salesCommissions>
                <dollarAmount>0</dollarAmount>
            </salesCommissions>
            <findersFees>
                <dollarAmount>0</dollarAmount>
            </findersFees>
            <clarificationOfResponse></clarificationOfResponse>
        </salesCommissionsFindersFees>
        <useOfProceeds>
            <grossProceedsUsed>
                <dollarAmount>0</dollarAmount>
            </grossProceedsUsed>
            <clarificationOfResponse></clarificationOfResponse>
        </useOfProceeds>
        <signatureBlock>
            <authorizedRepresentative>false</authorizedRepresentative>
            <signature>
                <issuerName>MERSANA THERAPEUTICS INC</issuerName>
                <signatureName>/s/ Julie A. Olson</signatureName>
                <nameOfSigner>Julie A. Olson</nameOfSigner>
                <signatureTitle>President &amp; CEO</signatureTitle>
                <signatureDate>2010-02-25</signatureDate>
            </signature>
        </signatureBlock>
    </offeringData>
</edgarSubmission>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----