Nucleophile specificity of subtilisin (subtilopeptidase A) was studied via acyl transfer reactions in acetonitrile containing piperidine and 10 vol% of water. Ac-Tyr-OEt was used as acyl donor and a series of amino acid derivatives, diand tripeptides of the general structure Xaa-Gly, Gly-Xaa, Gly-Gly-Xaa (Xaa represents all natural L-amino acids except cysteine) were used as nucleophiles. The nucleophilic efficiencies of these peptides were characterized by the values of the apparent partition constants, papp, determined from the HPLC analysis of the reactions. The order of preference for the P; position was estimated to be: Gly > hydrophilic, positively charged > hydrophobic, aromatic > negatively charged > Leu >>> Pro side chain. For the P; position the order of preference was: Gly > hydrophilic, charged > hydrophobic, aromatic > Pro side chain. The values of pappfor Gly-Gly-Xaa tripeptides cover a range of only two orders of magnitude, with lower nucleophile efficiency for those with hydrophobic amino acid residues in the p3 position. The dipeptide with Pro in P; did not react at all, but a tripeptide having Pro in p3 was a very good nucleophile. The negatively charged amino acid residues in the P; position result in very weak nucleophilic behavior, whereas the peptides with Asp or Glu in p2 and p3 are well accepted. Generally, peptides of the Gly-Xaa or Gly-Gly-Xaa series were better nucleophiles than peptides of the Xaa-Gly series. The length of the peptide chain or amidation of a-carboxyl function had no influence on nucleophilic behavior. No significant difference in nucleophile specificity between su btilopeptidase A and nagarse was observed. 1996 0