The influence of the radiosensitizer misonidazole on the effectiveness of several alkylating agents and cis-platinum against advanced solid murine tumors was investigated. Tumor regrowth delay, frequency of tumor regressions, and animal life span were used to evaluate misonidazole in combination with cyclophosphamide, L-phenylalanine mustard, 1-(2-chloroethyl)-3-trans-4-methylcyclohexyl)-1-nitrosourea, aziridinyl-benzoquinone, and cis-platinum. In the advanced M5076 ovarian carcinoma, misonidazole enhanced the activity of cyclophosphamide, L-phenylalanine mustard, 1-(2-chloroethyl)-3-trans-4-methylcyclohexyl)-1-nitrosourea, and aziridinyl benzoquinone, but not cis-platinum. In early B16 melanoma, misonidazole plus cyclophosphamide was no more effective than cyclophosphamide alone. In advanced Lewis lung carcinoma, misonidazole enhanced the antitumor activity of cyclophosphamide but not 1-(2-chloroethyl)-3-trans-4-methylcyclohexyl)-1-nitrosourea. Misonidazole, at 1000 mg/kg, increased the antitumor effectiveness of L-phenylalanine mustard and cyclophosphamide in M5076 tumors by factors of 2.2 and 1.8, but caused only a 1.2- and 1.3-fold increase in the myelotoxicity of these agents as determined by spleen colony assay of normal bone marrow. Misonidazole also increased the toxicity of cyclophosphamide and L-phenylalanine mustard in non-tumor-bearing mice but to a lesser degree than it enhanced antitumor activity. These results indicate that misonidazole is capable of enhancing the effects not only of ionizing radiation but of alkylating agents as well.