The hydrolysis of various oligopeptides in solution by intact Moniliformis moniliformis was examined using paper chromatographic analysis of the incubation medium. In the presence of transport inhibitors, the respective peptide sub-units and/or amino acid residues accumulated in the bathing medium. Only peptides with serine, methionine, leucine or alanine at the NH2-terminal end of the peptide were hydrolysed. There was no hydrolysis when these amino acids were located internally or at the COOH-terminus indicating genuine aminopeptidase activity of the class, α-aminoacylpeptide hydrolase. Hydrolysis was negligible when the NH2-terminus was arginie, aspartic acid, glutamic acid, glutamic acid, glycine, histidine, lysine, phenylalanine, proline, tryptophan, tyrosine, or valine. In separate experiments, mediated uptake of 0.1 mM3H-leucine by the worms in 2 min was inhibited 100% by 5 mM unlabelled leucine or tri-serine, but only partially inhibited by 5 mM Ser-Gly (66%), 10 mM Ser-Gly (74%), 5 mM Leu-Leu (69%), 10 mM Leu-Leu (70%), 5 mM Leu-Gly (58%) or 5nM Met-Met (69%). Bacause the inhibitions produced by 5 mM Leu-Leu plus 5 mM Met-Met (79%) or 5 mM Leu-Leu plus 5 mM Ser-Gly (76%) were not additive, a single enzyme is indicated. The name serine aminopeptidase is proposed because of its preference for serine.