Pancreatic glucagon (PG) and other glucagon-like immunoreactive materials (GLI) were measured in the plasma of normal and of newly diagnosed untreated diabetic children, using an antiglucagon serum (AGS) highly specific for pancreatic glucagon (AGS 18) and an AGS which crossreacts with extracts of intestinal mucosa (AGS 10). Gut GLI was considered to be the difference between "total" GLI (AGS 10) and PG (AGS 18). Glucose and immunoreactive insulin (IRI) were also measured. PG, total GLI and gut GLI were significantly elevated in children with severe insulin insufficiency and were reduced to normal by insulin treatment, even though a significant fasting hyperglycemia was still present. In three diabetic children who had high initial plasma IRI levels the three glucagon fractions were normal. We conclude that insulin insufficiency is characterized not only by high plasma levels of PG as previously reported, but also of gut GLI. These abnormalities can be corrected by the administration of insulin.