SummaryS-Adenosylhomocysteine synthase was present in extracts prepared from six lines of rat hepatoma. There was no apparent correlation between the specific activity of this enzyme and any of the other biological properties of the tumors. The presence of the hepatoma did not affect the activity of adenosylhomocysteine synthase in livers of host animals. Hepatic enzyme activity in both host and control rats showed an adaptive increase to an increase in dietary protein. In contrast, dietary protein failed to affect the specific activity of adenosylhomocysteine synthase in five hepatomas. Paradoxically, enzyme activity in hepatoma 7787 declined when the host rats were fed a high-protein ration.