**Exhibit 4.12****Amendment Agreement****to the Participation Agreements**     by and among     1. **tbg Technologie-Beteiligungs-Gesellschaft mbH,**Ludwig-Erhard-Platz 3, 53179 Bonn     - hereinafter also referred to as “**tbg**” -     2. **Micromet AG**, Staffelseestr. 2, 81477 Miinchen/Munich, eingetragen im Handelsregister desRegistergericht Miinchen unter/registered with the Commercial Register of the Munich Local Courtunder HR B 133 040,     - hereinafter also referred to as “**Micromet**” or “**TU**” -**Preamble**     A. tbg and Micromet have entered into a total of six participation agreements establishingsilent partnerships among tbg and Micromet. In detail the following contracts are concerned:       tbg No. Nominal Value Final Compensation***) Contract Start0268 321 / I 716.064,28  € 6 %**) 14./25.10.960268 322 / II 177.009,25  € 6 %**) 14./25.10.960268 323 / III 164.453,45  € 6 %**) 15.02./02.03.990268 324 / IV 476.348,66  € 6 %**) 15.02./02.03.990268 341 * / V 262.432,83  € 7 %***) 15.02./02.03.990268 342 * / VI 760.150,93  € 7 %***) 15.02./02.03.99       Total 2.556.459,40  € 0 0        * additional option right of tbg for conversion into shares. ** for the first 5 years 30 % flat, thereafter for each commenced contract year in % of thenominal value. *** for the first 5 years 35 % flat, thereafter for each commenced contract year in % of thenominal value.     B. The participation agreements were entered into among tbg, at that time acting underthe name of tbg Technologie-Beteiligungs-Gesellschaft mbH der Deutschen Ausgleichsbank, andMicro-met AG. The silent partnership agreements were registered with the Commercial Register on 7September 2000 pursuant to § 292 (2) No. 1 German Stock Corporation Act.     C. The shareholders of Micromet AG intend to contribute their shares in the Company toMicromet, Inc., with its registered seat in Delaware, USA. This company shall
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henceforth act underthe name Micromet, Inc., Delaware, USA. To this end, on 6 January 2006, Micro-met AG entered intoan Agreement and Plan of Merger and Reorganization with Micromet, Inc., Deleware, CancerVaxCorporation, Delaware and Carlsbad Acquisition Corporation, Delaware with the consent of tbg(hereinafter also referred to as “**Merger Agreement**”). The parties intend to amend the participationagreements in light of the forthcoming change of the shareholder structure.     NOW, THEREFORE, the parties hereby agree as follows:**§1****Repayment of the Participation Agreements I — IV**     1. The parties are in agreement that the nominal values of the Participation Agreements I — IVshall become due for repayment to tbg within 14 days after the consummation of the merger ofCarlsbad Acquisition Corporation with and into Micromet, Inc. as provided for in the MergerAgreement. Up to that date, the minimum compensation in the amount of 6.0 % p.a. gross shall bepaid in accordance with the agreements. Within 14 days after the consummation of the merger, TUshall pay to tbg an amount of in total EUR 2,000,000.00 gross in satisfaction of tbg’s claim forrepayment of the nominal values of the Participation Agreements I — IV and in satisfaction of allclaims of tbg for final compensation under the Participation Agreements I — IV as well as all otherclaims of tbg under the Participation Agreements I — IV. The parties are in agreement that afterthe payment of the minimum compensation under sentence 2 above and the payment of in total EUR2,000,000.00 gross under sentence 3 above, the Participation Agreements I -IV shall be finallysettled and no further claims of whatever kind among the parties out of or in connection with theParticipation Agreements I — IV, irrespective of their legal grounds, whether known or unknown,exist anymore.     2. The parties are further in agreement that the consummation of the transactions contemplatedin the Merger Agreement does not have any impact on the term of the Participation Agreements V andVI and that these accordingly have a fixed term up until 31 December 2008 at which point in timethey become due for repayment to tbg together with the final compensation agreed in the contracts.**§2****Profit and Loss Participation**     1. Sentence 3 of § 9 (4) of the Participation Agreements I — IV is deleted. The parties arein agreement that tbg does not participate in losses of neither TU nor Micromet, Inc.     2. This amendment shall apply for the first time to the current business year 2006.     3. Save as provided otherwise, the provisions of the Participation Agreements I -VI remainunaffected.
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**§3****Further Amendments**     § 12 (2) d) of the Participation Agreements I — IV as well as § 11 (2) c) of the ParticipationAgreements V- VI shall apply accordingly with regard to Micromet, Inc., Delaware, USA.**§4****Condition Precedent**     This amendment agreement shall come into force as soon as all of the following conditions arefulfilled:     a) Consent of the Shareholders’ Meeting of Micromet AG to the present amendment agreement;     b) Production of a guarantee declaration duly signed by Micromet, Inc. in accordance with theform attached as **Annex A**;     c) Contribution of more than 80 % of the shares in Micromet AG to Micromet, Inc.**§5****Consent**     tbg hereby grants its consent pursuant to § 5 of the Participation Agreements I — IV and § 4of the Participation Agreements V — VI to the contribution of the shares in Micromet AG inMicromet, Inc. as provided in detail in the Merger Agreement. After the consent of theShareholders’ Meeting has been granted, Micromet AG will cause the registration of this AmendmentAgreement with the Commercial Register without undue delay.**§6****Final Provisions**     1. Changes and amendments to this Agreement require the written form. Oral side agreements tothis Agreement do not exist.     2. In the event that a provision of this Agreement is invalid, the validity of the remainingprovisions shall not be affected thereby. TU and tbg are under an obligation to replace invalidprovisions of this Agreement by such valid provisions, which correspond to the intent and purposeof the invalid provisions to the closest possible extent.
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     3. Place of jurisdiction for all disputes out of this Agreement or its performance shall beBonn.     Bonn, 6 February 2006 Munich, the 30 January 2006       /s/ Michael Steinmetzer /s/ Gregor K. Mirow       (tbg) (Micromet)  
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**Exhibit4.12****Annex A****[Letterhead of Micromet, Inc., Delaware, USA]**tbg Technologie-Beteiligungs-Gesellschaft mbH