The gel filtration profiles of immunoreactive glucagon as measured by region-specific radioimmunoassays were studied in plasma samples from eight patients with glucagon-producing tumours and in extracts from five of these tumours, and compared with profiles in plasma samples from 24 normal subjects, and pancreas extracts from four patients without pancreatic tumours. In all extracts a component corresponding in size to the glucagon marker constituted the majority of the immunoreactivity, but small amounts of larger components were found in normal subjects as well as tumour patients. Plasma samples from both groups contained glucagon-sized as well as larger components with elution position corresponding to approximately 8,000 daltons. However, it was impossible to localize the source (pancreatic versus extra-pancreatic) of the latter forms. Thus gel filtration profiles do not distinguish patients with glucagonomas from normal, and are of no greater value than simple radioimmunological plasma concentration determination.