Cystatin, a cysteine proteinase inhibitor, was isolated from chum salmon (Oncorhynchus keta) pituitary glands by ion-exchange chromatography on Mono-Q, gel filtration on Superdex 75, and reverse-phase HPLC on an ODS following ethanol-ammonium acetate extraction. Salmon pituitary cystatin was equipotent to chicken egg-white cystatin in the papain inhibitory assay. The cystatin consists of 111 amino acid residues with two disulfide linkages formed between 66-75 and 89-109, and has 43% identical sequences with chicken egg-white cystatin with consensus sequences of reactive sites, Gly(4), Gln-X-Val-X-Gly (48-52), and Ile(Val)-Pro-Trp (96-98).