2 The free internal Ca] concentration in human red cells is set according to the leak- 2 and-pump principle: There is a finite passive Ca+ influx at the physiological 2 2 Ca+ -gradient across the membrane which is compensated by Ca+ pumping in the outward direction with a rate given by the degree of saturation of the A TP-fuelled Ca- 2 pump at the steady-state internal Ca+ concentration. Simons (1982) recently devised a method allowing the measurement...