A branched-chain amino acid aminotransferase was extracted from rumen ciliates of the genus Entodinium and was partially purified by Sephadex G-200, DEAE-cellulose and DEAE-Sephasex A-50 column chromatography. The purified enzyme was active only with leucine, isoleucine and valine, and required pyridoxal phosphate as cofactor. The amino acids competed with each other as substrates. The enzyme had optimal activity at pH 6.0 in phosphate buffer. The Km values for the substrates and cofactor are as follows: 1.66 for leucine; 0.90 for isoleucine; 0.79 for valine; 0.29 mM for alpha-ketoglutarate; and 0.1 muM for pyridoxal phosphate. Enzyme activity was inhibited by rho-chloromercuribenzoate and HgCl2. Gel filtration indicated the enzyme to have a molecular weight of 34,000.