The tripeptide Z-Cys(Bzl)-Tyr-Ile-OtBu (I) has been synthesized by papain, chymotrypsin and thermolysin catalysis using two different strategies: a) starting from the C-terminal amino acid and b) starting from the N-terminal amino acid. The optimum reaction conditions for obtaining the peptides Z-Tyr-Ile-OtBu (II), Z-Cys(Bzl)-Tyr-OtBu (III) and Z-Cys(Bzl)-Tyr-Ile-OtBu (I) were established after analyses of the effects of pH, reaction time, concentrations of buffer, enzyme and substrates, relative proportions of the carboxyl to amine components and nature of the organic solvent on the coupling yield. The highest yields obtained for II and I using chymotrypsin and papain as catalysts were 74% and 45%, respectively. Starting from the N-terminal amino acid, and using papain and thermolysin as catalysts, the yields obtained were 91% for III and 92% for I. The effects of amine- and carboxyl-protecting groups on the extent of dipeptide synthesis catalyzed by chymotrypsin were also studied.