Abstract Oral glucose-tolerance tests (G.T.T.s) with serum - immunoreactive - insulin (I.R.I.) measurements were done on a " diabetic " Tamil Indian family, on members of a Cape Coloured population who screened positive on urine and capillary-blood testing, and on controls. The more potentially diabetic a group became, the higher the fasting insulin level and the greater the insulin response, whether this potentiality was indicated by overweight, borderline G.T.T.s, or diabetic parentage, and however the insulin response was expressed. However, when chemical diabetes appeared, the I.R.I. response was reduced and delayed. It is concluded that, with regard to oral glucose stimulation in these two population groups, the earliest biochemical lesion is associated with insulin excess rather than insulin deficiency.