The effect of calcium and a soluble cytoplasmic activator on (Ca2+ + Mg2+)-ATPase of density-separated human red cells was investigated. At all calcium concentrations tested, dense (old) lysed cells and their isolated membranes displayed lower activities as compared to the light (young) cells and their membranes. Isolated membranes from all density red cell fractions showed two distinct (Ca2+ + Mg2+)-ATPase activities; one at low calcium and another at moderate calcium concentrations. At high calcium concentration, (Ca2+ + Mg2+)-ATPase activity of isolated membranes was low in all cell fractions. In contrast to the isolated membranes, lysed cells from all density fractions had a maximum (ca2+ + Mg2+)-ATPase activity only at a low concentration of calcium, while moderate and high calcium concentrations produced low activity. Upon isolation of membranes, a substantial loss of (Ca2+ + Mg2+)-ATPase activity took place from all density cell fractions. Upon membrane isolation, the relative loss of (Ca2+ + Mg2+)-ATPase activity at low Ca2+ concentration was greater in older cells. The extent of stimulation of (Ca2+ + Mg2+)-ATPase by the activator at low calcium concentration was 3-4-fold greater in older cell membranes than in the young ones. These data suggest that the lower (Ca2+ + Mg2+)-ATPase activity in old cells could be accounted for by a selective loss of (Ca2+ + Mg2+)-ATPase activity at low Ca2+ concentration presumably due to reduced affinity of old cell membranes to activator protein.