An investigation on circulating immune complexes (AgAb) was carried out in 80 diabetics with severe microangiopathy and in 71 diabetics without microvascular lesions. The duration of the disease, the type of diabetes, the type of treatment and the main localization of microangiopathy (retinopathy and nephropathy) were taken into account. AgAb were detected by two different methods: the solid phase Clq binding test (ClqSP) and the conglutinin binding test (KgBt). AgAb detected by ClqSP were increased both in prevalence and quantities in diabetics with severe microangiopathy regardless of the duration of the disease and the type of diabetes. Long standing diabetics without microangiopathy had similar prevalence of AgAb as normal controls. The presence of AGAb was not in correlation with the type of treatment and was similar in diabetics with retinopathy and in those with nephropathy. When AgAb were detected by KgBt, they were found with higher prevalence in diabetics than in normal controls but no correlation with microangiopathy was observed. AgAb, detected by KgBt, were higher in long standing type I diabetics. Since the two methods detect different AgAb it is concluded that AgAb present in diabetics seem to be heterogenous and part of them are related to the presence of microangiopathy.