**Exhibit 12.1**

**CERTIFICATION**

I, Dr. Greg R. Collier,certify that:

1.                                       Ihave reviewed this annual report on Form 20-F of ChemGenex PharmaceuticalsLimited;

2.                                       Basedon my knowledge, this report does not contain any untrue statement of amaterial fact or omit to state a material fact necessary to make the statementsmade, in light of the circumstances under which such statements were made, notmisleading with respect to the period covered by this report;

3.                                       Basedon my knowledge, the financial statements, and other financial informationincluded in this report, fairly present in all material respects the financialcondition, results of operations and cash flows of the company as of, and for,the periods presented in this report;

4.                                       Thecompany’s other certifying officer and I are responsible for establishing andmaintaining disclosure controls and procedures (as defined in Exchange ActRules 13a-15(e) and 15d-15(e)) for the company and have:

(a)                                  Designedsuch disclosure controls and procedures, or caused such disclosure controls andprocedures to be designed under our supervision, to ensure that materialinformation relating to the company, including its consolidated subsidiaries,is made known to us by others within those entities, particularly during theperiod in which this report is being prepared;

(b)                               [Paragraphomitted pursuant to SEC Release Nos. 33-8238 and 34-47986]

(c)                                  Evaluatedthe effectiveness of the company’s disclosure controls and procedures andpresented in this report our conclusions about the effectiveness of thedisclosure controls and procedures, as of the end of the period covered by thisreport based on such evaluation; and

(d)                                 Disclosedin this report any change in the company’s internal control over financialreporting that occurred during the period covered by the annual report that hasmaterially affected, or is reasonably likely to materially affect the company’sinternal control over financial reporting; and

5.                                     Thecompany’s other certifying officer and I have disclosed, based on our mostrecent evaluation of internal control over financial reporting, to the company’sauditors and the audit committee of the company’s board of directors (orpersons performing the equivalent functions):

(a)  Allsignificant deficiencies and material weaknesses in the design or operation ofinternal control over financial reporting which are reasonably likely toadversely affect the company’s ability to record, process, summarize and reportfinancial information; and

(b)  Any fraud, whether or not material, thatinvolves management or other employees who have a significant role in thecompany’s internal control over financial reporting.
|        | Date: December 22, 2006                                                                                                                                                                                                                                                    |
|--------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
|   |                                                                                                                                                                                                                                                                       |
|   | /s/ Dr. Greg R. Collier                                                                                                                                                                                                                                                    |
|   | Dr. Greg R. Collier                                                                                                                                                                                                                                                        |
|   |                                                                                                                                                                                                                                                                       |
|   | A signed original of this written statement required  by Section 302 has been provided to ChemGenex Pharmaceuticals Limited and  will be retained by ChemGenex Pharmaceuticals Limited and furnished to the  Securities and Exchange Commission or its staff upon request. |

 

 

 

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