The present investigation was dedicated to support biochemical interpretations of well-known long-term microvascular complications in diabetes. Provided the hypothetical correlation between erythrocyte membrane rigidity and increased intracellular calcium content holds true, a reduced Ca2+-Mg2+-ATPase activity in diabetic subjects could represent the underlying biochemical mechanism. Thus, we have compared basal and calmodulin-activated ATPase activity in healthy and diabetic volunteers. We could demonstrate a significant reduction of basal and stimulated enzyme activity in diabetic subjects. Furthermore, partial purification of calmodulin from erythrocytes of diabetic patients and healthy subjects yielded experimental evidence that reduced enzyme activity in diabetic volunteers is due to an altered basal activity as well as to a reduced stimulation by calmodulin.