The transport of radioactive sulfate in cat red cells has been studied. The rate constant for 35SO4 inward movement under steady-state conditions is 0.24 ± 0.02/hr. This movement was found to be sensitive to osmotic changes in cell volume and to the nature of anions in the incubation medium; it increases with increasing cell volume and decreases with decreasing cell volume. The anions SCN, NO3, and I were found to inhibit the uptake of 35SO4. Furthermore, 1-fluoro-2,4-dinitrobenzene at a concentration of 1 mM inhibits (>90%) this uptake. The inward movement of erythritol-14C shows qualitatively the same dependence on cell volume as 35SO4, but it is insensitive to the nature of the anion present in the bathing medium. It was also found that the usually observed inhibition of radioactive Na uptake by SCN in cat red cells can be reversed when cell volume is increased.

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