Glucagon-like immunoreactivity (GLI), which can be separated from glucagon by isoelectric focusing, was detected in partially purified canine pancreatic extracts. Like gastrointestinal GLI, this insular GLI reacts with cross-reaction antiserum 78J but not with glucagon specific antiserum 30K and has an isoelectric point (pI) of 9.5, whereas canine pancreatic glucagon has a pI of 6.25. When combined with glucagon, the GLI-glucagon mixture gives 78J assay values less than their calculated sum, suggesting that glucagon inhibits the reaction between GLI and this crossreacting antiglucagon serum and conceals it in glucagon-containing extracts.