Sera from 86 well controlled diabetics were examined for the presence of immune complexes. Thirty-six patients were receiving standard insulins, 19 monocomponent preparations, 24 oral hypoglycaemic agents and seven dietary restriction alone. Three methods were used to detect complexes: measurement of complement components, a Clq binding assay (BA) and the Raji cell radioimmunoassay (RIA). Complement components were normal in all patients. Eleven (31%) of the group on standard insulins had a positive Raji cell RIA; none had an abnormal Clq-BA. One patient on monocomponent therapy had a mildly positive Raji cell RIA; Clq-BA was negative in each patient of this group. Thirteen (54%) of the patients on oral hypoglycaemic agents were positive on one or both assays while one patient on diet alone was abnormal on both assays. These data show that immune complex production is common in both insulin-requiring and non-insulin-requiring diabetics and that this phenomenon is strikingly less frequent in patients on monocomponent insulins. Such observations could bear relevance to the pathogenesis of microvascular complications in diabetes.