ntracellular calcium (Ca) concentration in erythrocytes (RBCs) is controlled by a low passive influx through a relatively impermeable membrane and by active efflux catalyzed by Ca2+,Mg2+-ATPase. Since precipitation of alpha-globin chains in thalassemic RBCs may interfere with normal membrane function, we studied the RBC intracellular Ca content and the RBC membrane Ca2+,Mg2+-ATPase activity in two groups of patients with nonsplenectomized (n = 9) and splenectomized (n = 9) beta- thalassemia intermedia and in two groups of matched controls. The mean +/- SD Ca concentration in the nonsplenectomized (n = 12) and splenectomized (n = 6) controls were 6.1 +/- 6.0 and 5.8 +/- 3.4 mumol Ca per liter of RBCs, respectively, compared with 26.0 +/- 7.6 (P less than .001) and 85 +/- 24.4 (P less than .001) in the nonsplenectomized and splenectomized thalassemia patients, respectively. The mean +/- SD Ca2+,Mg2+-ATPase activity in the eight nonsplenectomized patients was 0.77 +/- 0.58 mumol inorganic phosphate (Pi) per milligram of protein per hour compared with 0.66 +/- 0.41 in the controls (P = NS). Similar values were obtained for the splenectomized patients and their controls. No correlation was found between either the intracellular Ca content or the Ca2+,Mg2+-ATPase activity with the peripheral nucleated RBC count. These findings suggest that there is a major defect in the membrane of the thalassemic RBC leading to an increased Ca content that is more pronounced in splenectomized patients.