The efflux of chloride-36 from human erythrocytes under steadystate conditions is a saturable process that is competitively inhibited by bicarbonate and noncompetitively inhibited by acetate. This chloride self-exchange flux is reversibly dependent on the pH of the medium between 5.7 and 9.6 with a maximum flux at pH 7.8. The increase in chloride flux between pH 5.7 and 7.8 is inexplicable by the fixed charge hypothesis. The interpretations are made that chloride transport in human erythrocytes is carrier mediated, that bicarbonate utilizes the same transport mechanism, and that the mechanism can be titrated with hydrogen ions into less functional forms for chloride transport.

4. The rate of self-exchange of bromide, thiocyanate, and iodide between human red cells and media was determined after washing and labelling cells in media containing 120 mm bromide, thiocyanate, or iodide respectively. The rate of self-exchange of the three anions were 12, 3, and 0 4 % of the rate of chloride self-exchange found in the chloride medium.5. The Arrhenius activation energies of the self-exchange of bromide, iodide, and thiocyanate were all between 29 and 37 keal/mole, the same magnitude as found for the self-exchange of chloride.6. Although approximately 40 % of the intracellular iodide and salicylate ions appeared to be adsorbed to intracellular proteins, the rate of tracer anion efflux followed first order kinetics until at least 98 % of the intracellular anions had been exchanged. MADS DALMIARK AND JENS OTTO WIETH 7. The self-exchange of salicylate across the human red cell membrane occurred by a different mechanism than the one utilized by the inorganic monovalent anions. The activation energy of salicylate exchange (13.2 kcal/mole) was significantly lower than that of inorganic anion exchange. Salicylate exchange increased with decreasing pH in contrast to the exchange of chloride, which decreases when pH is lowered.

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