Calf chymosin catalyzes peptide synthesis optimally at pH 4-5 giving satisfactory yields of methyl esters or p-nitroanilides of benzyloxycarbonyl tetra- to hexapeptides, provided that hydrophobic amino acid residues form the new peptide bond. The enzyme efficiency depends also on the nature of adjacent amino acid residues. As an aspartyl proteinase with characteristic specificity pattern chymosin would be useful for synthesis of middle length peptides.