Kinetic studies of [14C]HCO3- and [35S]SO4(2-) transport show the existence of a common transport system for the two anions on the plasma membranes of porcine epididymal sperm. Km value for bicarbonate (8.3mM) is within the range of physiological concentration of bicarbonate. The anion transport activity is dependent on the temperature but independent of co-existence of Na+. Its pH dependency and sensitivity to the several anion transport inhibitors are similar to those observed in erythrocyte. Anion transport activity decreases during sperm maturation in epididymis.