A collagenase from the pyloric caeca of yellow-tail, Seriola quinqueradiata, has been purified by affinity chromatography on a collagen-polyacrylamide gel column. The purified pyloric caecum collagenase was inhibited almost completely by diisopropyl fluorophosphate, soybean trypsin inhibitor, N-tosyl-L-lysy1chloromethane, and human serum, and less strongly by EDTA and N-tosyl-L-phenylalanylchloromethane. On the other hand, cysteine somewhat activated this enzyme activity. Disc electrophoretic patterns of collagen degraded by the action of this enzyme revealed a marked decrease in the original β component and the appearance of numerous new components beneath the original α and β components. These findings indicate that, despite certain differences, this enzyme is more closely related to crab hepatopancreas collagenase than to most known animal and human collagenases.