An acid proteinase from the culture filtrate of Penicillium camemberti was isolated in a two-step purification procedure by cation exchange chromatography and gelfiltration. The enzyme is an aspartic proteinase inhibited by pepstatin, DAN, and EPNP, with a molecular mass determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of 33.5 kDa. The optimum activity for hydrolysis of denatured hemoglobin is around pH 3.4. The enzyme is highly specific for the aromatic and hydrophobic amino acid residue in insulin B-chain and, like pepsin, selectively splits only one Leu7-Met8 peptide bond in the squash trypsin inhibitor CMTI 1. The hydrolyzed bond can be resynthesized by P. camemberti proteinase at neutral pH.