An increased catalase activity in Candida spp. has been suggested as a mechanism that reduces amphotericin B activity. Furthermore, resistance to antifungal agents like amphotericin B has been reported in some cancer patients undergoing chemotherapy treatment. In this study we analysed the influence of chemotherapy agents on catalase activity in Candida albicans, the major species involved in yeast infections. Eight strains of C. albicans isolated from HIV-positive patients were exposed to cyclophosphamide, cytarabine, dacarbazine and methotrexate antineoplastic drugs at the concentrations used during therapy. Catalase activity was measured and compared to the control group. Very significant differences (P < 0.01) were found when C. albicans was exposed to methotrexate (2 microg ml(-1) = 4 microM). For cyclophosphamide (50 microg ml(-1)), cytarabine (1 microg ml(-1)) and dacarbazine (8 microg ml(-1)), no differences were found (P > 0.05) between the control and drug-exposed groups. Although more extensive studies are necessary, these data do suggest that the antineoplastic drug methotrexate contributes to the resistance to antifungal drug therapy by varying catalase activity.