Soluble immune complexes (AgAb) as detected and quantitated by the solid phase Clq assay (Clq-SP) were found to be increased in (a) long-duration diabetics with proliferative retinopathy and (b) short duration diabetics with early onset of retinopathy irrespective of whether they were treated with insulin or oral hypoglycaemic agents (OHA), in comparison to a normal population. No such increases were observed in diabetics of comparable duration without retinopathy. The trend for long-term diabetics to show an increased prevalence of AgAb according to the severity of retinopathy was statistically significant. Detection and quantitation of AgAb by the Raji cell assay (RAJI) gave comparable results although the differences were less pronounced and fell short of statistical significance. AgAb as detected by either method in insulin-treated diabetics could not be correlated with insulin antibodies. These findings suggest that AgAb, not necessarily comprised of insulin and anti-insulin antibodies, may contribute to the pathogenesis of diabetic microangiopathy.