The extracellular pH (pHe) in many solid tumors is often lower than in normal tissues. Cells may survive conditions of acid pHe because antiports in their membrane exchange Na+ for H+, or HCO3- for Cl-, and thus regulate the intracellular pH (pHi). We have therefore assessed the effects of drugs which interfere with regulation of pHi on survival of Chinese hamster ovary and human bladder cancer MGH-U1 cells in tissue culture. Nigericin, an ionophore which acidifies the cytoplasm when cells are placed in medium at low pHe, was not toxic at pHe 6.5 or above but became very toxic as pHe was reduced below this value. Amiloride and 4,4'-diisothiocyanostilbene 2,2-disulfonic acid, inhibitors of the Na+/H+ and HCO3-/Cl- exchangers, respectively, decreased pHi in the presence of nigericin at low pHe. These drugs showed little or no toxicity in the pHe range of 6.0-7.0 but added greatly to the toxicity of nigericin. A combination of all three drugs led to toxicity in the pHe range of 6.5-6.8, well within the measured range of tumor pH, but not at pHe 7.0 or above. A combination of low pH and hypoxia, two conditions likely to be found in regions distant from tumor blood vessels, caused cell mortality in the absence of drugs, and this effect was increased by nigericin used alone or in combination with amiloride and 4,4'-diisothiocyanostilbene 2,2-disulfonic acid. These drugs may be regarded as prototypes for potential new anticancer agents that might achieve selective killing of tumor cells by interfering with the regulation of intracellular pH.