A purification method of kynurenine transaminase is described. The steps consist of pH 5 treatment, ammonium sulfate fractionation, heating at 60[degree] for 5 minutes, and hydroxyl apatite column chromatography. About 200-fold purification has been achieved starting from the supernatant of rat kidney homogenate. [alpha]-Ketoglutarate and pyridoxal phosphate have been found to afford considerable protection of kynurenine transaminase against thermal denaturation. Spectropho to metric measurements on the purified enzyme revealed the absorption bands at 280 and 410 m[mu]. This enzyme has an optimum at approximately pH 6.8. The effect of adding various reagents on the activity of the purified enzyme has been determined.