Antitumor activity of 6-thioguanine against L-1210 leukemia was studied in combination with various antitumor agents in clinical stages, such as mitomycin-C, carbazilquinone, cyclophosphamide, 3,3'-dimesyloxydipropylamine tosylate (864T), 4-hydroperoxyisophosphamide, and 3-[(4-amino-2-methyl-5-pyrimidinyl)methyl]-1-(2-chlorethyl)-1-nitrosourea hydrochloride (ACNU). The combination treatments with 6-thioguanine and each of six agents, especially with ACNU, showed a distinct therapeutic effect against the early L-1210 leukemia at dosage levels not producing any significant antitumor activity with each agent alone (ip-ip). The excellent antitumor activity of the combination of 6-thioguanine with ACNU was also proved in various sites of tumor inoculation and routes of drug administration systems, i.e., ip-iv, iv-ip, and iv-iv systems. Moreover, the effectiveness of the combination of these two drugs was clearly shown against advanced L-1210 leukemia which was refractory to each agent alone.