In order to establish a normal value of plasma glucagon immunoreactivity (GI) and glucagon-like immunoreactivity (GLI) during a newly adopted 75 g OGTT, 50 normals (N), 102 individuals with IGT and 20 diabetics (D) were subjected to the OGTT, and their plasma GI and GLI levels were determined at various intervals by radioimmunoassay using 2 kinds of the C-terminal region specific antibody, OAL123 and 30K, and of the antibody specific for the N-terminal and/or central region of glucagon, OAL196, respectively. The basal levels of OAL123-GI and 30K-GI and OAL196-GLI in the 3 groups were as follows; N, 114.3, 80.8, and 335.5; IGT, 107.6, 76.1, and 338.5; and D, 135.7, 76.9, and 342.2 pg/ml. After glucose administration, a significant decrease in plasma GI and increase in plasma GLI were observed in the 3 groups, although their changes from the basal levels were variable. The plasma samples of inexplicably high GI concentration were chromatographed to clarify the nature of the hyperglucagonemia. The apparent GI was mostly eluted in the Vo component, but negligibly at the 3500 mol.wt. glucagon fraction. There was a marked difference in the Vo peak depending upon the antiserum used. These facts suggest that plasma GI values are dependent on the amount of BPG present in particular samples, and the antibody used.