The specificity and mode of action of penicillolysin, a metalloproteinase from Penicillium citrinum, were investigated with several bioactive-oligopeptides. The enzyme showed a high affinity toward the Pro-X (X = Gln, Lys, Leu or Arg) bonds of substance P, dynorphin A (1-13), neurotensin and chicken brain pentapeptide, and the R-R bonds in dynorphin A and neurotensin. Preferential cleavage of bonds by the enzyme with hydrophobic amino acid residues at the P1 position were observed on the peptides used. The specificity of penicillolysin differs from that of other metalloproteinases. The M(r) and pI were determined as 18,000 and 9.6, respectively. The first 50 amino acids in the N-terminal region were TKETCSNASRKSALEKALSNTVKLANAAATAARSGSASKFSEYEKTTSSS. CD spectra on the hollo- and apo-enzymes of penicillolysin were studied.