**Exhibit 31.1**

**Certification ofPrincipal Executive OfficerPursuant to Section 302 of the Sarbanes-Oxley Act of 2002**

I, C. Randal Mills, certify that:

1.               I have reviewed this Quarterly Report on Form 10-Q of OsirisTherapeutics, Inc.

2.               Based on my knowledge, this report does not contain any untruestatement of a material fact or omit to state a material fact necessary to makethe statements made, in light of the circumstances under which such statementswere made, not misleading with respect to the period covered by this report; 

3.               Based on my knowledge, the financial statements, and otherfinancial information included in this report, fairly present in all materialrespects the financial condition, results of operations and cash flows of theregistrant as of, and for, the periods presented in this report; 

4.               The registrant’s other certifying officer(s), and I areresponsible for establishing and maintaining disclosure controls and procedures(as defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the registrantand have: 

(a)          Designed such disclosure controls and procedures, or caused suchdisclosure controls and procedures to be designed under our supervision, toensure that material information relating to the registrant, including itsconsolidated subsidiaries, is made known to us by others within those entities,particularly during the period in which this report is being prepared; 

(b)         [Paragraph omitted in accordance with SEC Release 34-47986]

(c)          Evaluated the effectiveness of the registrant’s disclosurecontrols and procedures and presented in this report our conclusions about theeffectiveness of the disclosure controls and procedures, as of the end of theperiod covered by this report based on such evaluation; and

(d)         Disclosed in this report any change in the registrant’s internalcontrol over financial reporting that occurred during the registrant’s mostrecent fiscal quarter (the registrant’s fourth fiscal quarter in the case of anannual report) that has materially affected, or is reasonably likely tomaterially affect, the registrant’s internal control over financial reporting;and 

5.               The registrant’s other certifying officer(s) and I have disclosed,based on our most recent evaluation of internal control over financialreporting, to the registrant’s auditors and the audit committee of theregistrant’s board of directors (or persons performing the equivalentfunctions): 

(a)          All significant deficiencies and material weaknesses in the designor operation of internal control over financial reporting which are reasonablylikely to adversely affect the registrant’s ability to record, process,summarize and report financial information; and

(b)         Any fraud, whether or not material, that involves management orother employees who have a significant role in the registrant’s internalcontrol over financial reporting.

Date: September 15, 2006
|        |   | /s/ C. Randal  Mills     |   |   |
|--------|--------|--------------------------|--------|--------|
|   |   | C. Randal Mills,  Ph.D.  |   |   |
|   |   | Chief Executive  Officer |   |   |

 

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