Peptidyl diazomethyl ketones and fluoromethyl ketones containing histidine in the C-terminal position were synthesized to determine their properties as proteinase inactivators. These were examined chiefly with derivatives of Z-Ala-His. The protection of histidine during conversion of the C-terminal residue to the diazomethyl ketone required unblocking conditions which avoid acid due to the lability of this function. This was achievable with a Cbz-imidazole derivative since aminolysis provided deblocking without disturbance of the diazomethyl ketone function. In the case of the fluoromethyl ketone synthesis using fluoroacetic anhydride (Dakin-West procedure), the desired product could be isolated without ring blocking. The Z-Ala-His products showed enhanced selectivity for inactivation of cathepsin B over L when compared to analogous dipeptide inhibitors.