Tyrosine aminotransferase (EC 2.6.1.5) of human liver was purified 2200-fold by successive chromatography on DEAE-cellulose DE-52, Ultrogel AcA-34, CM-Sephadex C-50 and hydroxyapatite to a specific activity of 64 units/mg of protein. The purified enzyme had a molecular mass of 95 500. The Km-values were 1.04 X 10(-3) mol/l, 0.17 X 10(-3) mol/l and 0.69 X 10(-6) mol/l for tyrosine, 2-oxoglutarate and pyridoxal 5'-phosphate, respectively. In the final purification step the enzyme activity was divided into two major fractions and a minor third one. On isoelectric focusing, three distinct fractions of specific tyrosine aminotransferase activity were obtained. The isoelectric points of these fractions were 4.9, 5.1 and 5.3, respectively. These findings imply that the human tyrosine aminotransferase consists of three subforms. No differences in properties studied could be found between the subforms. The coenzyme, pyridoxal 5'-phosphate, could be removed by dialysis.