1. A radio-immunoassay for gastrin has been developed using partially purified porcine gastrin to raise antibodies and highly purified natural porcine gastrin I for radio-iodination with (125)I. The separation of antibody-bound from free hormone was performed by a double-antibody method.2. In this assay highly purified natural porcine gastrin I, synthetic human gastrin I, radio-iodinated porcine gastrin I, gastrin in the plasma of a healthy volunteer, a patient with pernicious anaemia and another patient with the Zollinger-Ellison syndrome were immunologically identical.3. The fasting plasma gastrin concentration of fourteen gastric ulcer patients was significantly higher than that of the 113 hospital controls with no history of gastro-intestinal disease, while twenty-seven duodenal ulcer patients had gastrin levels within the normal range.4. Plasma gastrin concentration was significantly elevated in pernicious anaemia (fifty-one patients), achlorhydria (thirty-three patients), hypochlorhydria (eleven patients) and in nine patients with histologically proven Zollinger-Ellison syndrome.5. In human volunteers a protein meal stimulated endogenous gastrin release while a carbohydrate meal did not. Atropine sulphate I.M., and hydrochloric acid orally, produced a significant fall in the level of circulating gastrin.