Human platelets are known to have Fc receptors that are able to recognize soluble immune complexes and to respond to that stimulation by aggregating and releasing soluble factors. In diabetes, enhanced platelet aggregation has been proposed as one of the factors contributing to the development of microangiopathy. Soluble immune complexes isolated from seven diabetic patients were found to enhance ADP-induced platelet aggregation and release of ATP. This enhancement was proven not to be an artifact due to the isolation protocol, by comparison of purified immune complexes with nonspecific protein purified from normal sera by identical or slightly modified isolation protocols. Soluble immune complex appear to be the first well-characterized platelet aggregating factors form the sera of diabetic patients. The natural of the antigen involved in their formation does not appear relevant, since very similar results were obtained whether soluble immune complexes were purified from patients with insulin-anti-insulin complexes in their serum, or from those without such complexed but with positive results in nonspecific screening techniques.