Specific radioimmunoassays were used to quantify two basement membrane components, 7S collagen and laminin P1, in sera of 70 patients suffering from diabetes mellitus types I and II with and without clinical signs of chronic diabetic complications. Serum levels of both antigens were increased in diabetics compared to controls (p<0.001). Concentrations of 7S collagen were significantly different in diabetics with signs of microvascular damage compared to those without small vessel disease (p<0.05). The difference between laminin P1 concentrations in the two groups of diabetics was not significant (p<0.2). The augmented levels of circulating 7S collagen and laminin P1 may reflect alterations of basement membrane metabolism. Thus, the measurement of concentrations of these basement membrane components in serum may be a useful tool for monitoring the development of chronic diabetic complications.