Proinsulin levels (PLC) in serum were determined after gel filtration on specimens obtained during oral glucose tolerance testing (OGTT) in seven patients with repeated abnormality in OGTT and in a group of seven control subjects matched for age. Fasting and thirty-, sixty-, and 120-minute postglucose venous samples were analyzed for glucose (PG) and immunoreactive insulin (IRI) as well as PLC. PG and IRI mean concentrations were greater at all testing times in the patient population, but the mean IRI/PG was significantly higher in the patients only at fasting. PLC mean levels were higher in the patients but not to a level of significance. Percentage of total IRI attributable to PLC at each time point was identical between the two groups. The apparent diminished effectiveness of circulating insulin in chemical diabetes cannot be attributed to an abnormal proportion of proinsulin.