A systematic study was made of the ability of aminopeptidase N from Lactococcus lactis subsp. cremoris Wg2 to hydrolyse different peptide substrates. The enzyme showed a marked preference for substrates containing arginine as the N-terminal residue but, to a lesser extent, was also capable of cleaving other residues such as lysine and leucine. There was a tendency for the activity to increase with the hydrophobicity index of the C-terminal residue of dipeptide substrates. It was also observed that the enzyme tended to have higher affinities but lower Vmax values for tripeptides with hydrophobic C-terminal residues. The values determined for Km and Vmax increased with chain length for oligopeptides of the general formula Lys-Phe-(Gly)n, the optimum, as determined from Vmax/Km, being when n = 4. Typical Km values for the most effective substrates were in the range 0.2-0.6 mM.