Two major obstacles in the treatment of patients with central nervous system malignancies are drug resistance and host toxicity. The goal of combination chemotherapy is to achieve therapeutic effects that are more favorable than using a single drug alone, but without an increase in normal organ toxicity. The study reported here examined the combination of a topoisomerase I inhibitor, irinotecan (CPT-11), with three different alkylating agents: 1,3-bis(2-chloroethyl)-1-nitrosourea, busulfan, and cyclophosphamide. We evaluated the antitumor effects of these three combinations against a panel of human tumor xenografts derived from central nervous system malignancies, including adult high-grade gliomas (D-54 MG, D-245 MG) and a childhood ependymoma (D-612 EP). In replicate experiments, the alkylating agents were given on day 1 in doses varying from 10% to 75% of the dose lethal to 10% of the animals, and CPT-11 was given on days 1-5 and 8-12 in doses varying from 10% to 100% of the dose lethal to 10% of the animals. The antitumor effects of the various combinations ranged from less than additive (7.61 days below additive with 0.5 CPT-11 + 0.75 cyclophosphamide in D-54 MG) to statistically significant (P < 0.001) supraadditive effects (18.80 days above additive with 0.5 CPT-11 + 0.5 1,3-bis(2-chloroethyl)-1-nitrosourea in D-54 MG). These studies show that the combination of the topoisomerase inhibitor CPT-11 and alkylating agents may increase the antitumor effect in some cases well above additive with no increase in host toxicity (0/10 deaths in both experiments cited above) and should be considered for combination chemotherapy of central nervous system malignancies.