Plasma was filtered on G50 Sephadex, and two components of circulating insulin, “little” insulin and “big” insulin, were measured by immunoassay. The former component is indistinguishable from insulin, whereas the latter more closely resembles proinsulin and the other insulin-like substances isolated from the pancreas. In thin normal subjects, the fraction of plasma insulin that was big insulin (per cent “big”). 15-30 min after oral glucose, was less than 5%; per cent big rose 2- to 8-fold over the next hour and by 90-120 min represented 5-29% of the plasma insulin. In young thin subjects with idiopathic glucose intolerance associated with normal concentrations of plasma insulin, an identical pattern of big insulin was observed. In thin subjects in whom elevations of per cent big at 90-120 min during the standard test were only modest, starvation for 48 hr before the glucose administration resulted in a more pronounced rise in the per cent big insulin. Early after glucose administration to obese and acromegalic subjects, the per cent big was higher than in thin subjects. The magnitude of the elevation was roughly correlated with elevations in the fasting plasma insulin concentration. By 90-120 min, the per cent big in obese and acromegalic patients was the same range as in the thin subjects. Intravenously administered arginine and tolbutamide in a small number of subjects yielded a response that was similar to oral glucose; the per cent big was low early after stimulation and increased with time. In two patients with islet cell carcinomas, the per cent big was in the same range as in normal subjects.