The synthesis of difluoromethylene ketone retroamides, e.g. I [R1 = CHMe2, CH2CHMe2, CH2Ph, cyclohexylmethyl, CH2C6H4NH2-4, CH2C6H4[NHC(NH2):NH]-4, (CH2)4NH2, CH2C6H4OCH2Ph-4; R2 = MeO2CCH2CH2-Ala-Ala-Pro, Boc-Phe-Nva, Z-Val, Me2CHCH2CO-D-Phe-Pro, H-D-Phe-Pro; R3 = Ac, PhCH2CO-D-Val, PhCH2NHCOCH(CHMe2)CO, Ph(CH2)4CH(CH2CHMe2)CO, Me2CHCH2CO, H], is described.Several examples of application to aspartyl or seryl proteases illustrate the versatility of this inactivation concept.