The molecular weights of orni-thine-ketoacid aminotransferase in pure form and in crude extracts of rat liver were compared. The molecular weight of the pure enzyme was concentration dependent. At concentrations of 10 mg/ml its molecular weight was 202,000 dal-tons, but this value was reduced to 116,000 daltons when the enzyme was diluted and allowed to equilibrate at a concentration of 0.1 mg/ml. The crude enzyme preparation also had a molecular weight of 116,000 daltons. The molecular weight of the enzyme subunit was 52,000 daltons and the pure enzyme contained one pyridoxal phosphate moiety per subunit. The data suggest that the enzyme may exist as a dimer in vivo.