The growth of experimental tumors was found to involve an intensification of acetylation of sulphadimidine. Inhibition of carcinosarcoma Walker-256 by 88% and sarcoma-180 by 36% on treatment with 60 mg/kg body weight of hydrazine sulphate was followed by decreases in the rate of acetylation of 39.4 and 29.4%, respectively. When treatment failed to suppress tumor growth (sarcoma-45), acetylation level showed no decrease. The results point to a correlation between growth of tumors and their inhibition on the one hand and the level of acetylation on the other.