Antibody was readily produced in a rabbit against synthetic porcine secretin coupled to BSA. The final dilution of the antiserum to bind 50% of 1 fmol 125I-labeled secretin was 1 : 150,000. The effective equilibrium constant (Keff) according to Scatchard was 3.4 X 10(11) l/mol, the average equilibrium constant (Ko) according to Sips 3.5 X 10(11) l/mol, and the index of heterogeneity (alpha) according to Sips 1,00. No cross reactivity was found for gastrin, glucagon and insulin. 125I-labeled synthetic porcine secretin was prepared by the Chloramine-T-method, and the label purified on a Sephadex G-15 column followed by a SP Sephadex C-25 column had a specific radioactivity of 1.150 muCi/nmol. The radioimmunoassay method described has a detection limit of 1.6 pmol/l with 95% confidence limit, a within assay precision of 9,6%, and a between assay precision of 12.8%. It allows detection of fasting plasma secretin in the low pmol/l range, and the rather sharp rise and fall in plasma secretin subsequent to a brief period of duodenal acidification. The problem involved in measuring plasma secretin have been overcome by acidification of the plasma, and by subtracting the "apparent" secretin concentration in corresponding secretin-free plasma prepared by incubation at 37 degrees C for 96 h for each subject.