The viability of cells within the acidic microenvironment found in solid tumours is expected to depend on the regulation of intracellular pH (pHi). 5-(N,N-hexamethylene) amiloride (HMA) is a potent inhibitor of the Na+/H+ antiport, a major mechanism for the regulation of pHi. We have therefore studied the cytotoxicity of HMA in combination with nigericin, a cell-acidifying agent, for EMT-6 cells in monolayer cell culture, in spheroids and in a murine tumour model. The combination of nigericin and HMA was toxic to cells in tissue culture at extracellular pH (pHe) < or = 6.8 (as may be found in tumours) but not at pH 7.0 or above (as in most normal tissues). Compared with amiloride, the relative potency of HMA in causing in vitro cytotoxicity (approximately 100-fold) was similar to that for inhibition of the Na+/H+ antiport. The fluorescent probe Hoechst 33342 was used with flow cytometry to study the cytotoxicity of HMA and nigericin at different depths in multicellular tumour spheroids. Only small differences in the level of cell survival were observed, but higher concentrations of HMA were required as compared with those giving equal levels of survival in monolayer culture. The pharmacokinetics of HMA in mice was studied by using high-performance liquid chromatography: after intraperitoneal injection of 20 micrograms g-1, the plasma level of HMA peaked at 8 microM after about 15 min and decreased to 1 microM at 120 min; the half-life was 35 min. Nigericin and HMA, at doses of 1.25 micrograms g-1 and 10 micrograms g-1 respectively, failed to cause significant cell killing in the EMT-6 murine tumour, but the surviving fraction was reduced to approximately 0.004 when hydralazine was administered with nigericin and HMA. Local tumour irradiation (15 Gy), followed by treatment with these drugs, led to cell killing that was additive to the effects of drugs and radiation alone, so that hypoxic cells which survived radiation did not appear more sensitive to pH-dependent drug treatment. Acid-mediated therapy can lead to cell death in murine solid tumours, but further measures will be required before the strategy can be exploited clinically.