Intact human erythrocytes can be readily loaded with Ca by incubation in hyperosmotic media at alkaline pH. Erythrocyte Ca content increases from 15-20 to 120-150 nmol/g Hb after incubation for 2 h at 20.degree. C in a 400 mosmol/kg, pH 7.8 solution containing 100 mM sodium chloride, 90 mM tetramethylammonium chloride, 1 mM potassium chloride and 10 mM calcium chloride. Ca uptake is a time-dependent process that is associated with an augmented efflux of K. The ATP content in these cells remains at more than 60% of normal and is not affected by Ca. Ca uptake is influenced by the cationic composition of the external media. The response to K is diphasic. With increasing K concentrations, the net accumulation of Ca initially increases, becoming maximal at 1 mM K, then diminishes, falling below basal levels at concentrations above 3 mM K. Ouabain inhibits the stimulatory effect of low concentrations of K. The inhibitory effects of higher concentrations of K are ouabain insensitive and independent of the external Ca concentration. Na also inhibits Ca uptake but this inhibition can be modified by altering the external concentration of Ca. The efflux of Ca from loaded erythrocytes is not significantly altered by changes in osmolality, medium ion composition or ouabain. Hypertonicity increases the net uptake of Ca by increasing the influx of Ca; some part of the Na-K transport system is involved in this influx process.