A sensitive, specific and reproducible radioimmunoassay for secretin is described. Antibodies were readily produced against low microgram quantities of synthetic secretin. The secretin antibodies did not cross-react with the structurally similar G.I.P., V.I.P., or glucagon. Synthetic secretin was iodinated using Chloramine "T" and purified by a two-stage procedure incorporating gel filtration and gradient elution from a cation-exchange column. Plasma samples were found to produce variable interference in the assay necessitating the incorporation of secretin-free "blanks" for each patient's plasma. Production of secretin-free plasma was by incubation of plasma samples at 37°C for 96 hours. The sensitivity of the assay was 12.5-25 pg/ml. Normal fasting secretin levels were 21 ± S.E. 7 pg/ml. A mean rise in plasma secretin to 220 pg/ml was observed after intraduodenal acidification.