It is one of the characteristic features in diabetes mellitus that insulin response to glucose is low. Nine low insulin responders with normal glucose tolerance in whom prediabetes mellitus should be suspected were found to be included in 104 apparently healthy subjects (9%). A 100 g glucose tolerance test (GTT) was performed in nine apparently healthy subjects with a low insulin response and 11 healthy subjects with a normal insulin response. A 50 g GTT was performed in 15 normal subjects, 20 subdiabetics, 38 chemical diabetics and 17 mild diabetics. In all subjects C-peptide during GTT was estimated by the double antibody systems using a synthetic human C-peptide. Though C-peptide secretion from the beta cell of the pancreas seems to keep pace with insulin secretion, the peak of C-peptide was delayed in its appearance in the circulating blood, as compared with that of insulin. The pattern of plasma C-peptide during GTT in diabetics was identical with that of insulin. In the low insulin responders C-peptide response to glucose was observed to be lower like that of insulin than in the normal insulin responders. However, a molar ratio of C-peptide to insulin at 30 min during GTT in the former was higher than that in the latter.