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The specific activities of TS and TK determined in the cytosols of Ehrlich and L1210 ascites tumor cells changed significantly during their logarithmic growth. In both tumors, the highest activity of TK was in early log phase while that of TS occurred in late logarithmic growth. The activity curves of TS were practically the same and those of TK were similar in the two tumors; however, the absolute values of the latter were different. TK/TS activity ratios in the early and late log growth of Ehrlich tumor were 20 and 2, in those of L1210 tumor 4 and 0.15 respectively. MTX cytotoxicity was considerable throughout the logarithmic growth of L1210 tumor, significantly higher values were observed however when TS activity increased. The variable degree of MTX cytotoxicity indicated an intracellular manifestation of the changes in TS activity determined in vitro. The different patterns for the specific activities of TS and TK, as well as, the parallel changes of TS activity and MTX cytotoxicity during tumor growth may be useful information for the antimetabolite research carried out with experimental tumors.