Abstract: (1) A combination of daily dosage at 50mg/kg of stilbestrol diphosphoramide with 6-mercaptopurine at 30mg/kg/day exhibited a greater effect against mouse Crocker sarcoma 180 than each drug administered separately at the same dose level. A similar result was also obtained in the case of rat Jensen sarcoma. (2) Thiocit decreased the toxicity of nitrogen mustard in mice and rats, it reduced, however, the antitumor effect of the latter concurrently. (3) The lethal effect of 6-mercaptopurine in mice was not influenced by L-cysteine at a single dose of 500mg/kg, although the decrease of white blood cell counts in rats caused by a sublethal dose of 6-mercaptopurine was counteracted by L-cysteine.