The inhibition of normal human placental alkaline phosphatase, its rare "D-variant" phenotype, and the "Nagao isoenzyme" found in an ovarian cancer fluid by peptides containing L-leucine were studied. The Nagao isoenzyme and the D-variant phenotype were distinct from the normal placental enzyme, but similar to each other in their inhibition by L-leucine and dipeptides containing L-leucine in their amino termini. On the other hand, L-leucinamide, L-Leu-L-Leu-L-Leu and L-Leu-Gly-Gly inhibitions did not readily discriminate between D-variant and normal phenotypes, but did so between D-variant and Nagao isoenzymes. The distinctions among these enzymes in their responses to peptide inhibitors may reflect differences in their primary structures.