The complete amino acid sequences of two isomeric endogenous inhibitors, IA-1 and IA-2, both of which specifically inhibit an intracellular serine proteinase (proteinase A) purified from the fruiting bodies of a higher basidiomycete, Pleurotus ostreatus, were determined. Both inhibitors are acidic polypeptides with respective molecular masses of 8307 and 8244 Da, as determined by plasma desorption mass spectral analyses, and their N-terminal serine residue is blocked by acetylation. The fragments generated from the inhibitors by proteolytic and chemical cleavages were subjected to amino acid composition, sequence, and mass spectral analyses. The sequence and molecular mass information for the peptides established that the inhibitors both consisted of 76 amino acid residues and differed from each other in that aspartic acid and glutamic acid residues at residues 12 and 15 of IA-1 were replaced by glycine and aspartic acid in IA-2, respectively. The molecular masses of IA-1 and IA-2 were calculated to be 8309 and 8237, based on the sequence data. The action of carboxypeptidase Y on IA-1 resulted in a complete loss of the inhibitory activity along with successive release of glutamine and threonine from the C-terminus. Cyanogen bromide cleavage of Met38-Pro39 and Met41-Lys42 in IA-1 and hydroxylamine degradation of IA-2 completely abolished their inhibitory activity. These results suggest that the whole molecules of both inhibitors are essential to their inhibitory activities. Their structural resemblance to propeptides of subtilisin family proteinases revealed their mechanism of action.