A radioimmunoassay (RIA) for human group I pepsinogens (PgI) in serum was developed, using PgI purified from gastric mucosa. The sensitivity (0.7 micrograms/l) and reproducibility of the assay were satisfactory for clinical use. In normal controls, total serum pepsinogen (T-Pg) level was 58.9 +/- 31.7 micrograms/l (mean +/- SD) (PgI, 43.6 +/- 25.0 micrograms/l; PgII, 15.3 +/- 11.1 micrograms/l). Peptic ulcer cases had elevated T-Pg levels (gastric ulcer, gastroduodenal ulcer and duodenal ulcer, in increasing order of magnitude). T-Pg levels were not useful for diagnosis of peptic ulcer because of a large overlap with normal controls. T-Pg levels were low in patients with gastric polyp and in aged subjects. In these groups, the decrease of PgI was more marked than that of PgII.