Fifteen 1-peptidyl-2-haloacetyl hydrazines, which can be considered halometanes of azapeptides containing Phe in P2 and alpha-aza-Ala or alpha-aza-Gly in P1, were synthesized and tested as models of cysteine-proteases inhibitors. By use of kinetic methods, they proved to irreversibly inactivate papain and cathepsin B via a reversible enzyme-inhibitor intermediate. Second-order rate constants of inactivation in the range 26-23000 M-1s-1 were observed for papain and 2000-39600 M-1s-1 for cathepsin B. KI for the reversible EI adducts ranged from 230 to 0.16 microM for papain and from 11 to 0.37 microM for cathepsin B. Structure of possible reversible EI complex is proposed and used to discuss the effects of structural variation of the inhibitors on the kinetic parameters of inactivation. Title compounds proved to be selective for cysteine-proteases, since no inhibiting activity could be detected toward trypsin, chymotrypsin and porcine pancreatic elastase at 0.1 mM concentration, after 6 h incubation. Relatively low aspecific alkylating properties were also verified in tests using glutathione as the nucleophile.