The killing of metazoan parasites by eosinophils involves the activation of a respiratory burst oxidase. To investigate whether human eosinophils possess an H+ conductance that might participate in the extrusion of H+ generated by the respiratory burst, we employed the whole cell patch-clamp technique under conditions designed to isolate putative H+ currents. We observed a slow activation of outward currents by depolarizing voltage steps. The reversal potential (Erev) of the currents was a function of the H+ gradient, demonstrating that the current was carried by H+. The H+ conductance was activated by cytosolic acidification and reversibly blocked by divalent and trivalent cations. During large prolonged depolarizing voltage steps, the current activation was followed by a decrease in current. This was due to cytosolic H+ depletion, as evidenced by 1) a change in Erev and 2) a cytosolic alkalinization. We also observed a rundown of the current, possibly due to the loss of a cytosolic factor necessary for H+ current activity. An elevated pipette Ca2+ concentration (1 microM) activated the H+ conductance, suggesting that the cytosolic Ca2+ concentration is involved in the physiological regulation of H+ currents. The Ca(2+)-activated currents had properties similar to the currents observed at low Ca2+ concentrations (Erev, high-affinity block by Zn2+, kinetics of tail currents, kinetics of rundown). The Ca2+ effect might be mediated by phospholipase A2, inasmuch as 1) the currents were also activated by arachidonic acid, 2) the Ca2+ effect and the arachidonic acid effect were not additive, and 3) the Ca2+ effect, but not the arachidonic acid effect, was blocked by a phospholipase A2 inhibitor. Taken together, our results demonstrate that human eosinophils have large H+ currents that are activated by physiological intracellular signals. The electrophysiological properties of the H+ currents and their regulation strongly suggest that they participate in H+ extrusion during the respiratory burst.