Cancer procoagulant (CP) is a cysteine proteinase from cancer cells that initiates blood coagulation. Members of two classes of unique and highly specific cysteine proteinase inhibitors, peptidyl diazomethyl ketones (PDK) and peptidyl sulfonium salts (PSS), were studied to determine whether or not they inhibited CP. The inhibitors did not impair the activity of the coagulation system. There was a differential inhibitory effect of the 6 PDK and 2 PSS inhibitors, influenced by the amino acid composition or sequence of the peptide moiety, that suggests differences in structural features of the active site of CP and papain. CP was inhibited by both classes of inhibitors.