The gastric hormone gastrin is produced in multiple forms that vary in their biological properties. In this analytical review, the strategies available for the assay of different gastrins in plasma are considered. Except for research purposes, it is seldom necessary or even desirable to employ assays that are specific for an individual molecular form of gastrin. Instead, routine clinical assays of plasma gastrin should ideally react equally with the sulphated and unsulphated forms of the COOH-terminally amidated peptides, the most important of which are peptides of 17 and 34 amino acid residues (i.e. G17 and G34), and should not react with the related hormone cholecystokinin. Methods appropriate for the use of such assays are reviewed. These assays are important in the diagnosis of gastrinoma. Although it is recognized that circulating concentrations of gastrin are elevated in other conditions, including chronic atrophic gastritis, Helicobacter pylori infection and long-term administration of proton pump inhibitors, it is not clear whether gastrin radioimmunoassay is important for the clinical management in these conditions.