ATPase activity and phosphorylation by [y-32 P ] ATP of isolated plasma membrane of alveolar macorphages are stimulated in a parallel fashion by physiologic concentrations of Ca2+, with half-maximal activating effect of this ion at (3-7) X M. For various membrane preparations, a direct proportionality exists between Ca2+-dependent ATPase activity and amount of 32P incorporated, Labeling of membrane attains the steady-state level by 1 0 sec at O'C, and is rapidly reversed by adenosine diphosphate (ADP). K' decreases the amount of membrane-bound 32 P, mainly by enhancing the rate of dephosphorylation of the 32P-intermediate. Hydroxylamine causes a release of about 905% of 32 P bound to the membrane, thus indicating that the 32P-intern~ediate contains an acyl-phosphate bond. When the labeled plasma membrane is solubilized and electrophoresed on acrylamide gels in the presence of sodium dodecyl sulphate, the radioactivity appears to be largely associated with a single protein fraction of 132,500 ? 2,000 apparent molecular weight. These features of the macrophage Ca2+-ATPase suggest that the enzyme activity might be part of a surface-localized Ca*+-extrusion system, participating in the regulation of Ca2+-dependent activities of the macrophage.