Plasma group I pepsinogen (PG I) concentration is now a readily available non-invasive marker of gastric secretory function, with several potentially useful applications. Ninety-eight percent of control subjects had a plasma PG I below 115 ng/ml while values above this level were seen in 43% of duodenal ulcer patients. Plasma PG I levels below the observed normal lower limit of 30 ng/ml were seen consistently in patients with documented achlorhydria or pernicious anaemia. In 48 patients with a variety of upper gastrointestinal disorders there was a significant correlation between PG I pepsinogens and pentagastrin-stimulated maximal acid output. Changes in plasma PG I promise to be useful in evaluating both operations on the vagus and newer anticholinergic drugs, whilst in epidemiological and family studies these measurements seem well suited to explore duodenal ulcer heterogeneity.