Cytoplasmic tyrosine transaminase (L-tyrosine: 2-oxoglutarate aminotransferase, EC 2.6.1.5) was partially purified from normal, host liver and three Morris hepatomas and subsequently resolved by electrophoresis on polyacrylamide gels. Enzyme activity was detected histochemically in situ on the gels. Seven enzymatically active forms were detected in normal liver. The presence of growing hepatomas altered significantly the expression of this enzyme. Only one, two and four activity peaks were detected in the host liver of animals with highly (most liver-like), well and poorly (least liver-like) differentiated hepatomas, respectively. Similarly, only one, four and six peaks were detected, respectively, in highly, well and poorly differentiated hepatomas.