Abstract Thickening of the capillary basement membrane is the fundamental morphological alteration of diabetic microangiopathy. Leucocyte neutral proteases can degrade basement membrane in vivo. The activity of a purified neutral protease from granulocytes is inhibited by α 2-macroglobulin. It is suggested that the reported increase in the concentration of α 2-macroglobulin in the blood of diabetic patients inhibits the activity of leucocyte neutral proteases which normally degrade basement membrane. The resultant decreased rate of breakdown would contribute to the increase in basement-membrane material seen in diabetes.