A study of Ca++, Mg++-ATPase activity was carried out in normal (HHm) and diabetic Nigerians of both sexes with insulin-dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). The results showed that protein concentration of erythrocyte ghost membranes of healthy humans (HHm) was the highest when compared with protein concentrations of IDDM and NIDDM patients. The protein concentration was lowest in IDDM, while the value in NIDDM was between those of HHm and IDDM. The basal activities of erythrocyte Ca++-ATPase from IDDM and NIDDM were determined and were found to be significantly lower than that of HHm. The addition of calmodulin (CaM) 2 microg/ml stimulated the activity of the calcium pump in all the groups (IDDM, NIDDM and HHm). The effects of calcium (Ca++) and adenosine triphosphate (ATP) on the activity of the pump from each group were determined. Enzyme kinetics (Km and Vmax) revealed that the activity of Ca++, Mg++-ATPase was initiated by ATP in the presence of Ca++ in a dose-dependent manner. Calmodulin also enhanced the activity of the enzyme in the presence of Ca++ in all the groups, though activities in IDDM and NIDDM were significantly lower than in HHm. There was no significant difference in the activities between IDDM and NIDDM. These results suggest a defective calcium translocating mechanism in diabetic Nigerians.