1. The properties of the Ca-dependent K transport system of human red cell ghosts have been examined under equilibrium exchange conditions. 2. K transport is stimulated half-maximally by about 0-4muM-Ca2i+ or 5muM-Sr2i+, but much higher concentrations of Ba2i+ give only slight stimulation. Mg is a weak antagonist to Ca. 3. The free Ca2+ concentration in human red cells is estimated to be below 0-25muM. 4. The curve relating the rate of K transport to the intracellular Ca2+ concentration is complicated and suggests that internal Ca acts at three or more sites. 5. K, Rb and possibly Cs ions are transported by the Ca-dependent system. Under comparable conditions the relative rates are 1(K):1-5(Rb): less than 0-05(Cs). 6. No Ca-dependent transport of Na, Li or choline could be detected. If Na is transported, it must be at less than 1/40 of the rate of K. 7. The rate of K transport is almost linearly related to the K concentration in the 0-200 mM range, but the curve is sigmoid close to the origin. 8. Intracellular, but not extracellular Na inhibits K transport, in a way that suggests competition with K at more than one site. 9. These results suggest that the transport system has a complex mechanism.
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