The S-adenosylmethionine synthetase activities of rat liver and Novikoff ascites tumor have been partially purified and characterized by chromatographic behavior, kinetic analysis, sulfhydryl dependency, and response to inhibitors. We have shown that the tumor contains a single form of the enzyme, with a Km (methionine) of 21 muM, and that the liver contains two isofunctional forms, a minor form with a Km (methionine) of 21 muM, as well as a major form with a Km of 1 mM. The tumor contained more of the low Km form of the enzyme than the liver, although the total enzyme activity of liver (measured at high substrate concentrations) exceeded that of the tumor severalfold. The tumor enzyme also corresponded to the minor form of liver enzyme in elution position from Sephadex G-150 and diethylamino-ethyl cellulose, and both had a Km (adenosine 5'-triphosphate) of 0.14 mM. The tumor enzyme differed from the major form of the liver enzymes in elution position, and the Km (adenosine 5'-triphosphate) for the latter was 1.5 mM. In contrast to the major liver enzyme, the tumor enzyme did not appear to require sulfhydryl agents for the activity to be detected, was inhibited by S-adenosylmethionine, and was inhibited to a greater degree by tripolyphosphate. It is suggested that the two forms of the enzyme are involved in the production of S-adenosylmethionine for different biological functions, and their different properties may allow selective inhibition of tumor growth by chemotherapeutic agents.