The intracellular Ca2+ concentration of nearly all cells is kept at submicromolar levels. The magnitudes of transmembrane Ca2+ movement that maintain this steady state in the human red blood cell have long been debated. Although there is agreement that the physiologic extrusion of Ca2+ by the well-characterized Ca2+ ATPase amounts to 45 µmol/liter cells per h (1982. Nature (Lond.). 298:478-481), the reported passive entry rates in physiological saline (2-20 jjimol/liter cells per h) are all substantially lower. This discrepancy could be due to incomplete inhibition of the pump in the previous measurements of Ca2+ entry. We therefore examined both rate and mechanism of entry after completely inactivating the pump. This required pretreatment with iodoacetamide (to lower the intracellular ATP concentration) and vanadate (to inhibit any residual Ca2+ pump activity). The rate of Ca2+ entry (53 µmol/liter cells per h) was now found to be comparable to the accepted extrusion rate. Entry closely obeyed Michaelis-Menten kinetics (Kmax = 321 ± 17 nmol Ca/g dry wt per h, Km = 1.26 ± 0.13 mM), was competitively inhibited by external Sr2+ (Ki = 10.8 ±1.2 mM), and was accelerated by intracellular Ca2+. 45Ca2+ efflux from these pump-inactivated cells was also accelerated by either external Ca2+ or Sr2+. These accelerating effects of divalent cations on the opposite (trans) face of the membrane rule out a simple channel. Substrate-gated channels are also ruled out: cells equilibrated with 45Ca2+ lost the isotope when unlabeled Ca2+ or Sr2+ was added externally. Thus, passive Ca2+ movements occur predominandy by a reversible carrier-mediated mechanism for which Sr2+ is an alternate substrate. The carrier’s intrinsic affinity constants for Ca2+ and Sr2+, 1.46 and 0.37 mM-1, respectively, indicate that Ca2+ is the preferred substrate.