A trypsin inhibitor was isolated and purified from the bran of rice, Oryza sativa, by extraction with 1% sodium chloride, heat treatment, ammonium sulfate precipitation, ion-exchange chromatog-raphy on a CM-Sephadex C-25 and gel filtration on a Sephadex G-75. The final preparation was homogeneous by electrophoretic analysis. Rice bran trypsin inhibitor (RBTI) had a molecular weight of about 14, 500 and an isoelectric point of 8.07. The amino acid composition was characterized by high contents of basic amino acids, aspartic acid, glutamic acid, proline and cystine. BRTI inhibited bovine trypsin at an inhibitor-enzyme molar ratio of 1:1.6. It displayed, however, nobility to inhibit α-chymotrypsin, pepsin, papain and subtilisin BPN'.