The sidedness of proton modulation of K-Cl cotransport (K-Cl COT) was studied in low K sheep red blood cells stripped of cellular Mg, Mgi, at alkaline medium pH, pHo, by the divalent ionophore A23187 and a chelator. This procedure activates K-Cl COT, presumably, by inhibition of MgATP-dependent kinases. Ouabain-resistant K efflux and Rb influx were measured in Cl or NO3 either at variable pHi and fixed pHo, or vice versa, in erythrocytes pH- and volume-clamped with the anion exchange inhibitor 4,4'-diisothiocyanato-2,2'-disulfonic stilbene (DIDS) and sucrose. Between pHi 9 and 6, and at constant pHo 9, K effluxes decreased hyperbolically in Cl and linearly in NO3 whereas Rb influxes fell almost linearly in Cl and asymptotically in NO3. Thus, saturation of outward and inward K-Cl COT, the calculated difference of the fluxes in Cl and NO3, occurred at slightly different pHi values. Hill plots revealed pKa values of 6.5 and 7.0, and Hill coefficients of > 1 for outward and inward K-Cl COT, respectively. Raising pHo from 6 to 9 at fixed pHi slightly increased K and Rb fluxes in both Cl or NO3, but not K-Cl COT. The histidine reagent diethylpyrocarbonate (DEPC) inhibited low Mgi-activated K-Cl COT at approximately 4 mM, an effect partially reversible by subsequent treatment with hydroxylamine. It is concluded that protons inhibit erythrocyte K-Cl COT through internal histidine(s) which may be part of a pH sensor.