Pb" is thought to enter erythrocytes through anion exchange (AE) and to remain in the cell by binding to thiol groups. To define the role of AE mechanism and thiol groups in Pb2+ toxicity, we studied the effects c f drugs and conditions that modify AE and that modify thiol groups on the ability of Pb" to stimulate potassium efflux as measured with "Rb. The most potent stimulation of "' Rb efflux by Pb2+ occurred when conditions were optimal for the AE mechanism-that is, when bicarbonate was included in the buffer or a buffer made with Nal or NaCl rather than NaCIO, or NaN03 was used. Furthermorme, 4,4'-diisothiocyanatostil- bene-2,2'disulfonic acid and 4-acetamido-4'-isothiocyanatostilbene-2,2~-disulfonic acid, potent inhibitors of the AE mechanism, cclmpletely inhibited stimulation of the R6Rb efflux by Pb2+. These conditions or inhibitors did not affect stimulation of the "' Rb efflux by ionornycin plus Ca2'. A role for Caz+ channels was dismissed because the inorganic Caz+ channel tllockers, Cd2+ or Mn2+, did not prevent stimulation of "Rb efflux by Pb" but did inhibit stimulation by ionomycin plus Ca2+. 86Rb efflux was more sensitive to Pb2+ if erythrocytes were treated for 15 min with thiol-modifying reagents that enter cells, such as iodoacetamide, N-ethylmaleimide, or dithiothreitol, than to reduced glutathione, a thiolmodifying reagent that is not permeable to the cell. lhus, in erythrocytes the AE mechanism and internal thiol groups are critical factors that affect the stimulation of a Ca2+-dependent process by Pb2+.