Radiation inactivation was applied to analyze the molecular weight of the functional unit of (Ca2+ + Mg2+)-ATPase in human erythrocyte membranes. The enzyme activity was stable for at least 7 days at room temperature in membranes lyophilized in the presence of sucrose (150-300 mM). The enzyme activity in the lyophilized membranes and remaining after irradiation from a 60Co source was activated by calmodulin. A Mr of 290,000 +/- 15,000 was determined for (Ca2+ + Mg2+)-ATPase activity. Since the Mr by SDS-polyacrylamide gel electrophoresis is approximately 138,000 (Niggli et al. (1979) J. Biol. Chem. 254, 9955-9958), our results suggest that the Ca2+ pump ATPase functions as a dimer in the native human erythrocyte membrane.