Circulating autoantibodies to insulin can be detected in patients with insulin-dependent (type I) diabetes mellitus (IDDM) at the onset of the clinical disease. To characterize the autoantibody response in IDDM patients, we determined the frequency of circulating B cells committed to the production of IgM, IgG, and IgA to insulin in 12 newly diagnosed IDDM patients and, for comparison, in 9 healthy subjects and 17 insulin-treated IDDM patients. We found that B cells committed to the production of anti-insulin IgG, but not IgM, autoantibodies are present at much higher frequency in the circulation of newly diagnosed IDDM patients before insulin treatment (0.209 +/- 0.142%, mean value +/- SD of total IgG-producing cell precursors) as compared with age-matched healthy controls (0.032 +/- 0.030% of total IgG-producing cell precursors). In IDDM patients who had been treated with insulin, cells producing IgG antibody to insulin were 0.177 +/- 0.139% of total IgG-producing cell precursors. Generation of IgG mAb from B cells of IDDM patients revealed that they were monoreactive, i.e., they bound to insulin, but to none of the other Ag tested, and displayed a high affinity for insulin (Kd approximately 10(-7) moles/liter). In contrast, the IgG mAb derived from healthy subjects were polyreactive, i.e., they bound to all Ag tested, and displayed a low to moderate affinity for insulin (Kd approximately 10(-5) to 10(-6) moles/liter). These findings show that lymphocytes committed to the production of high affinity IgG autoantibodies to insulin are common in the B cell repertoire at the onset of IDDM.