alpha-Aminoadipate aminotransferase and kynurenine aminotransferase activities from rat kidney are reportedly associated with the same protein. We observed that when the supernatant fraction was maintained at pH 4.5 for 75 min, 100% of kynurenine aminotransferase activity was lost, whereas only 40% of aminoadipate aminotransferase activity was lost. We purified alpha-aminoadipate aminotransferase and kynurenine aminotransferase from rat kidney supernatant fraction to electrophoretic homogeneity by ammonium sulfate fractionation, DEAE-Sephacel, and hydroxylapatite chromatography. Kynurenine aminotransferase activity was precipitated by pH treatment. The remaining aminoadipate aminotransferase activity was concentrated and injected into rabbits to raise antibodies that were used to prepare an affinity column. A mixture of aminoadipate aminotransferase and kynurenine aminotransferase activities obtained after hydroxylapatite chromatography was subjected to affinity chromatography. Aminoadipate aminotransferase and kynurenine aminotransferase activities resolved as separate peaks, providing evidence that the two activities are associated with two different proteins.