Combination therapy with anti-HIV drugs and opportunistic infection drugs is a common practice in treatment of AIDS patients. Although toxic effects of most individual therapies are known, the toxic potential of most combination therapies has not been established. To understand the toxic consequences of combination therapies, the commonly used anti-HIV drug 3'-azido-3'-deoxythymidine (AZT) and tuberculosis infection therapies pyrazinamide, isoniazid, and rifampicin were evaluated by 13-week gavage studies in B6C3F1 mice, either alone or AZT in combination with one of the antituberculosis drugs. The doses include AZT 100, 200, and 400; pyrazinamide 1000 and 1500; isoniazid 50, 100, and 150; and rifampicin 100, 200, and 400 mg/kg/day. AZT alone caused hematopoietic toxicity with dose-related bone marrow suppression, macrocytic anemia, and thrombocytosis. Pyrazinamide or isoniazid alone at the doses tested did not cause significant toxicity. Rifampicin alone caused hematopoietic toxicity and possibly mild hepatic toxicity. Pyrazinamide below 10 times the therapeutic dose when given with AZT did not increase the hematological toxicity of AZT. Isoniazid markedly increased the hematological toxicity of AZT and contributed to mortality at 3 to 4 times the therapeutic dose combinations. Administration of rifampicin with AZT at the calculated therapeutic doses resulted in toxicity of far greater magnitude than that caused by AZT or rifampicin alone. Combination treatment with AZT and rifampicin caused severe anemia with mortality at 2 to 4 times the therapeutic dose combinations. However, AZT did not enhance the hepatotoxicity of rifampicin. Increased hematopoietic toxicity of AZT when given in combination with the above antituberculosis drugs may be due to changes in the metabolism of AZT. Results of these studies indicate that toxicological effects of combination therapies could be considerably more severe than and different from the toxicity of individual therapies.