We studied binding of 125I-labelled truncated-glucagon-like peptide-1 (proglucagon 78–107 amide) to a cloned rat insulin-producing cell line, RIN 5AH, in monolayer culture. Interaction of the peptide with pancreatic insulinoma cells was saturable and time dependent. Half-maximal binding was obtained when the cells were incubated in the presence of 3.3 × 10 −9 mol/l unlabelled truncated-glucagon-like peptide-1 (proglucagon 78–107 amide). Neither glucagon, full-length glucagon-like peptide-1 (proglucagon 72–107 amide) nor gastric inhibitory peptide competed for binding in concentrations up to 10 −6 mol/l.