The Cucurbita maxima trypsin inhibitor CMTI-III molecule was used as a vehicle to design and synthesize a series of trypsin chromogenic substrates modified in position PI: Ac-Ala-Val-Abu-Pro-X-pNA, where X = Orn, Lys, Arg, Har, Arg(NO2), Cit, Hei, Phe(p-CN), Phe(p-NH2); pNA = p-nitroanilide. The most active compounds (as determined by specificity constant k(cat)/K-m) were peptides with the Arg and Lys residues in the position discussed. Changes in the length and the decrease of the positive charge of the amino acid residue side chain in position P-1 resulted in the decrease or loss of the affinity towards bovine beta -trypsin. Among peptides containing amino acid residues with uncharged side chains in position P1, only one with p-cyano-L-Phe revealed activity. These results correspond well with trypsin inhibitory activity of CMTI-III analogues modified in the equivalent position, indicating the same type of interaction between position Pl of the substrate or inhibitor and S1 site specificity of trypsin.