Two trypsin inhibitor types, PII and PI II, were isolated by affinity chromatography of a potato extract on a column of trypsin immobilized on Sepharose 4B. Fraction PI I afforded after ion exchange chromatography on SE-Sephadex two isoinhibitors, PI IA (Mr approximately 18 000; pI approximately 6.3) and PI IB (Mr approximately 19 500; pI approximately 7.2). The chromatography of fraction PI II on SE-Sephadex yielded three inhibitors of approximately equal molecular weight (Mr approximately 13 500), PI IIC (pI approximately 6.3), PI IID (pI approximately 7.7), and PI IIE (pI approximately 9.1). All the inhibitors isolated show a high activity toward trypsin, acrosin, and chymotrypsin. Unlike the two isoinhibitors of PI I, which practically do not inhibit kallikrein, inhibitors PI II show an effect on this enzyme. Neither the isoinhibitors of PI I nor inhibitors PI II are active toward cathepsin D.