T he mechanism of endocytosis in resealed human erythrocyte ghosts was studied. The energy for endocytosis or micropinocytosis appears to be derived from Mg-ATP, and membrane internalization is preceded by activation of a membrane-associated Ca,Mg-ATPase and by the active efflux of Ca. Endocytosis, Ca,Mg-ATPase activity, and active Ca efflux all require the presence of Mg. Furthermore, these three phenomena, endocytosis, Ca,Mg-ATPase activity, and active Ca extrusion, all have a concentration dependence on Ca such that low concentrations stimulate and higher concentrations inhibit the phenomena. The optimal concentration of Ca is identical for endocytosis, active Ca efflux, and Ca,Mg-ATPase. Morphologic studies indicated that while active Ca efflux and activation of the Ca,Mg-ATPase activity occurred promptly upon onset of incubation, there was a significant time delay before endocytosis occurred, which suggests that endocytosis additionally involved a more slowly functioning mechanicochemical mechanism. Ruthenium red, a specific inhibitor of Ca,Mg-ATPase and Ca transport, inhibited endocytosis in a concentration-related manner. Prostaglandins E1 and E2 had no measurable effect on ghost endocytosis, active Ca efflux, or Ca,Mg-ATPase activity.