AbstractThe synthesis of L‐leucyl‐sarcosyl‐L‐phenylalanine is described. This tripeptide resists hydrolysis by the aminopeptidases AP I and AP II fromBacillus stearothermophilus. The peptide is a reversible inhibitor of both enzymes. The inhibition constant for AP I is 5 × 10−3mol/l and 1.5 × 10−3mol/l for AP II.In the hydrolysis of leucine p‐nitroanilide we observed noncompetitive inhibition by leucylsarcosyl‐phenylalanine with both enzymes.With dipeptide substrates however, the inhibitory effect of leucyl‐sarcosyl‐phenylalanine was drastically reduced by saturating sub