Pseudomonas aeruginosa elastase was used for peptide-bond synthesis with benzyloxycarbonylalanine and amino acid amides as nucleophilic substrates. Dipeptide-bond synthesis was observed only for hydrophobic amino acid amides. The rate of peptide synthesis, measured by h.p.l.c., was in the decreasing order: Phe > Leu > Tyr > Val, Ile > Ala, which is consistent with the decreasing order of hydrolysis rates of the corresponding tetrapeptides Ala-Ala-Xaa-Ala. In contrast with thermolysin, Ps. aeruginosa elastase permits the synthesis of tyrosine-containing peptides with tyrosine in the P'1 position. Furthermore, the rates of synthesis for other hydrophobic amino acid amides are higher with elastase than with thermolysin.