Gastrin antisera were raised by immunization of rabbits or guinea pigs with synthetic human gastrin I conjugated to bovine serum albumin by carbodiimide. Radioiodination of SHG: 2-17 was performed by the chloramine T-method and by an enzymatic procedure. AE-cellulose was used to get a monoiodinated tracer hormone. Antibody reactions with the different forms of gastrin and with CCK-PZ was characterized. Precision (VK = 6-8%) and reproducibility (VK less than 15%) of the gastrin values were comparable to the insulin assay. Gastrin stimulates the parietal cell and has trophic effects on gastric mucosa. Hypergastrinaemia in combination with hypersecretion exhibits clinical significance in patients suffering from Zollinger-Ellison-syndrome or excluded antrum-syndrome which are due to autonomous gastrin release. Some findings suggest a pathogenetic role of gastrin in duodenal ulcer disease. Those disease in which gastrin determinations are of clinical value are discussed.