Voltage-activated proton currents are reported for the first time in human peripheral blood T and B lymphocytes and in the human leukaemic T cell line Jurkat E6-1. The properties of H⁺ currents studied using tight-seal voltage-clamp recording techniques were similar in all cells. Changing the pH gradient by one unit caused a 47 mV shift in the reversal potential, demonstrating high selectivity of the channels for protons. H⁺ current activation upon membrane depolarisation had a sigmoidal time course that could be fitted by a single exponential equation after a brief delay. Increasing pH_o shifted the activation threshold to more negative potentials, and increased both the H⁺ current amplitude and the rate of activation. In lymphocytes studied at pH_i 6.0, the activation threshold was more negative and the H⁺ current density was three times larger than at pH_i 7.0. Increasing the intracellular Ca²⁺ concentration to 1 µM did not change H⁺ current amplitude or kinetics detectably. Extracellularly applied Zn²⁺ and Cd²⁺ inhibited proton currents, slowing activation and shifting the voltage-activation curve to more positive potentials. The H⁺ current amplitude was 100 times larger in CD19+ B lymphocytes and in Jurkat E6-1 cells than in CD3+ T lymphocytes. Following stimulation with the phorbol ester PMA, the H⁺ current density in peripheral blood T lymphocytes and Jurkat T cells increased. In contrast, the H⁺ current density of phorbol acetate (PMA)-stimulated B lymphocytes was reduced and activation became slower. The pattern of expression of H⁺ channels in lymphocytes appears well suited to their proposed role of charge compensation during the respiratory burst.