We had a patient with asymptomatic hyper-immunoreactive glucagonemia and with no evidence of pancreatic tumor detected by radiological examinations. The glucagon level was not decreased by the administration of glucose or somatosatin analogue (SMS 201-995). Gel filtration studies revealed that most glucagon immunoreactivity was eluted at the position of 150,000 daltons [big plasma glucagon (BPG)]. Binding studies with 125I-glucagon showed that glucagon autoantibody was negative. Acid treatment of plasma and reduction of immunoglobulin G (IgG) did not result in a shift of BPG to normal glucagon (3485 daltons). Glucagon immunoreactivity determined with anti-glucagon antiserum OAL 123 (C-terminal specific antiserum used in the present radioimmunoassay kit) did not dilute out in parallel to normal glucagon (3485 daltons), and the plasma glucagon level was normal with Unger's 30K (anther C-terminal specific antiserum) and OAL 196 (N-terminal specific antiserum). The patient's IgG dose-dependently reduced the binding of 125I-glucagon to anti-glucagon antiserum OAL-123. Glucagon degrading activity (GDA) was negative in the patient's plasma. These results suggest that the patient's IgG cross-reacted with the present anti-glucagon antiserum OAL 123, and caused a spuriously high plasma immunoreactive glucagon level.