Since several anticancer drugs are known to become more cytotoxic to cells in an acidic milieu, we have attempted to utilize the carbonic anhydrase inhibitor, Acetazolamide, to acidify the blood and tumor of C3H mice bearing the FSaIIC fibrosarcoma in order to sensitize tumor cells in vivo to CDDP, Melphalan, BCNU, SR4233 or PtCl4 (Fast Black)2 +/- hyperthermia. The direct cytotoxic interactions between the anticancer drugs and Acetazolamide were tested in FSaIIC cells in vitro with the monacidifying diuretic Chlorothrozide as a control. When cells were exposed to CDDP both diuretics protected against cytotoxicity in a dose dependent fashion. In contrast, cells exposed to Melphalan were minimally sensitized and those exposed to BCNU, SR4233, or PtCl4 (Fast Black)2 were essentially unaffected by the presence of the diuretic agents. Both diuretics were essentially non-toxic to cells in vitro, and, interestingly, both drugs markedly protected cells against hyperthermia under low pH conditions. In vitro, however, Acetzolamide produced a tumor growth delay of 2.3 days alone when given at 10 mg/kg i.p. once (the most effective dose) and produced additive growth delays with CDDP and Melphalan, but probably greater than additive delays with SR4233 and PtCl4 (Fast Black)2. When Acetazolamide was given daily for 5 days starting on the day the anticancer drugs were given once (day 7) essentially no further increase in tumor growth delay of nearly 16 days was observed versus only 4,6 days for the drug alone. When hyperthermia (43 degrees C min.) was delivered locally to the tumor after i.p. injection of the drugs, further growth delays were produced for every drug combination which probably were additive in extent. Blood and urine pH determinations revealed that a pH drop of 1 units occurred in the blood and a pH elevation of 1 to 21 units occurred in the urine 1 hr. after i.p. injection of Acetazolamide. These results indicate that this carbonic anhydrase inhibitor can add to the anticancer activity of the drugs tested. The mechanism may involve its ability to acidify the intratumoral environment, but other mechanisms can not be excluded.