We previously reported the presence of a carrier-mediated sulfate transport system in human lung fibroblasts (IMR-90) (A. Elgavish, J.B. Smith, D.J. Pillion, and E. Meezan. J. Cell. Physiol. 125: 243-250, 1985). Kinetic studies carried out in the lung fibroblasts show that Cl- inhibits SO42- uptake in a competitive manner. Taken together with the fact that high extracellular Cl- stimulates SO42- efflux, these results suggest that SO42- uptake into lung fibroblasts occurs via a SO42--Cl- exchange mechanism. Extracellular HCO3- inhibits sulfate influx in a competitive manner (pH 7.5) but has no marked effect on sulfate efflux. SO42- and HCO3- may therefore have the ability to bind to a common extracellular anion binding site, but they do not appear to exchange for one another. Lowering extracellular pH has a stimulatory effect on the initial rate of sulfate uptake. The pK of the extracellular pH effect is around pH 7.0, indicating that small changes in the extracellular pH around the ambient levels encountered under physiological conditions will markedly affect sulfate influx into the cell. Kinetic studies suggest that lowering extracellular pH increases the initial rate of sulfate influx by increasing the affinity of the carrier for sulfate twofold. Lowering intracellular pH inhibits the initial rate of sulfate influx into the cell. The pK of this intracellular pH effect is around pH 7.0, indicating that physiological levels of intracellular protons are necessary for the normal activity of the anion exchanger. These studies indicate that sulfate transport in lung fibroblasts can be attributed to a SO42--Cl- exchanger, the affinity for SO42- of which is increased by extracellular protons.