The coupling between Cbz-Phe-OH and Leu-NH2 catalyzed by thermolysin was examined under various experimental conditions. The highest yield (ca. 80%) was obtained in the reaction mixture containing 0.05 M each of the carboxyl and amine components and 10 muM enzyme at pH 7 and 37 degrees C for 5 h. The reactivity was ca. 100 times higher than that of alpha-chymotrypsin. Amino acid derivatives or peptides were useful as amine components, though a hydrophobic or bulky amino acid residue was required at the N-terminal position. Strict stereospecificity was observed at this position. A hydrophobic or bulky amino acid residue occupying the C-terminal position of carboxyl components was also favorable for synthesis. The specificity requirements for synthesis were the same as those for hydrolysis.