Antitumor activities of a combination chemotherapy with a water-soluble nitrosourea, 3-[(4-amino-2-methyl-5-pyrimidinyl) methyl]-1-(2-chloroethyl)-1-nitrosourea hydrochloride (ACNU), and a single dose of 6-thioguanine were studied using three obstinate murine tumor systems, i. e., Lewis lung carcinoma, B16 melanoma, and an advanced stage of L1210 leukemia systems. Therapeutically synergistic effect was observed either definitely against 1- or 2-day-old Lewis lung carcinoma and 6-day-old L1210 leukemia or moderately against 1-day-old B16 melanoma. Single intravenous treatment on day 7 after subcutaneous implantation of Lewis lung carcinoma, when the tumors had already metastasized to the lungs, produced a significant regression of tumor and a significant increment in survival time of tumor-bearing mice. In comparative studies, the combination of ACNU and 6-thioguanine showed a greater and a wider spectrum of antitumor activities against these tumors than those obtained by the combination with ACNU and a single dose of 5-fluorouracil, methotrexate, or 6-mercaptopurine. Increment in lethal toxicity for normal and tumor-bearing mice was not observed by the combination of ACNU and 6-thioguanine in contrast to definite increases in this toxicity by the combination of ACNU and 5-fluorouracil. The present experimental results may suggest the clinical utility of the combination chemotherapy with ACNU and 6-thioguanine in the treatment of several solid tumors as well as acute leukemias.