There has been described elsewhere (1, 2) a permeability phenomenon analogous in certain respects to chemical catalysis but involving a diffusion process rather than a chemical reaction. It now appears that this phenomenon of "catalyzed diffusion," as it may be called for brevity, is of wider applicability than was at first suspected. The present paper deals with certain recently investigated aspects of this question.The work had its origin in the observation of 0rskov (3, 4) that the rate of hemolysis of mammalian erythrocytes in solutions of ammonium chloride can be enormously increased--that is, 50 times or more--by the addition of a little bicarbonate. 0rskov in his first paper (3) suggested that carbonic acid has a specific effect of some sort on the erythrocyte which makes it more permeable to anions; later (4) he modified this view and postulated instead an increased permeability of the cell to the ammonium ion. For various reasons which have in part been set forth elsewhere (5) neither of these explanations seems to be satisfactory, and there has been proposed in their place the principle of catalyzed diffusion (2), in which the action of the bicarbonate is believed to be not primarily upon the cell at all but rather upon the solutions on the two sides of the cell membrane.In Fig. 1 A is represented the mechanism (6) for the entrance of NH4C1 into the erythrocyte, which in spite of certain objections (3, 4) that we believe to be answerable (5), seems best to explain the known facts. According to it NHn molecules first enter the cell and there become converted into NH4 ions; a subsequent shift of anions through the anion-permeable membrane completes the process. The overall rate of transfer of the salt is slow, chiefly because of the extremely low concentration of OH' ions in the cell at any given time.If, now, NHj-ICO~--or any other bicarbonate--be added to the external solution, we have the conditions represented in Fig. 1 B in which a new molecule of very great penetrating power, CO2, is present. Mass law considerations demand that for equilibrium the product in.i~ X [HCOa]ia~ide must

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