The aim was to compare the three molecular forms of plasma immunoreactive gastric inhibitory polypeptide (IR-GIP) i.e. void volume (Vo), 8 and 5 kDa IR-GIP, found in type 1 diabetics with those found in normal subjects. Plasma from 6 non-fasting newly diagnosed ketotic type 1 diabetics obtained before and 1 h after a test meal given at start of insulin treatment, and before and 1 h after a test meal given after one and seven days of insulin treatment, respectively, was gel filtered and so was plasma from 6 normal subjects. The immunoreactivity in the effluents was measured with five different antisera. The elution positions of the three peaks were similar in controls and diabetics. With any given antiserum none of the components differed significantly as to amount of immunoreactivity between diabetics and controls, neither after the meals nor in the fasting state. The amount of Vo did not change in response to the meal, whereas the 8 and 5 kDa forms in the diabetics increased similarly to the increase in normals, also during ketosis. The Vo component did not differ significantly between diabetic and normal subjects, but it decreased significantly after start of insulin treatment. In the non-fasting, ketotic state before start of insulin treatment, no IR-GIP form was elevated significantly above normal postprandial levels. We conclude that the molecular forms of IR-GIP are similar in type 1 diabetics and normal subjects, but the molecular forms measured and their relative amounts vary according to which antiserum is used. The present study does not support that lack of insulin and ketosis markedly influence IR-GIP in plasma.