We examined whether swelling-activated K-Cl cotransport is electrogenic in human erythrocytes. Baseline membrane potential, measured by the change in fluorescence of the carbocyanine dye diS-C3-5, was not different in hypotonically swollen (-7.6 mV) or isosmotically swollen cells (-9.5 mV). We used hemisodium, a new highly selective Na ionophore, in varying concentrations, in the presence of a fixed outwardly directed Na gradient (intracellular Na, 75 mM; external Na, 1 mM) to vary membrane potential over a wide range despite identical K and Cl concentrations. The membrane potential varied between -8 and -90 mV. K influx increased slightly with hyperpolarization in swollen and nonswollen cells. However, the difference between the two fluxes, swelling-activated K influx, a measure of K-Cl cotransport, was unaffected by voltage changes, as was swelling-activated K efflux. We conclude that K-Cl cotransport in human erythrocytes is electroneutral and by inference has a 1:1 stoichiometry.