The serum values of PG I and gastrin have been established in a normal population and in several clinical diseases. The PG I is raised in duodenal, gastric, and pyloric ulcer even though the gastrin is normal. Both PG I and gastrin values are raised in renal insufficiency and the Zollinger-Ellison syndrome. The PG I is lowered in atrophic gastritis and alcoholic cirrhosis, and is at the limit of detection in Biermer anemia and total gastrectomy. Insulin and sham-feeding are stimulants for PG I release by patients with duodenal ulcer, but no correlation is observed between PG I output and PAO in the studied group. The results show that PG I is able to distinguish between associated hypergastrinemia and hypoacidity (Biermer anemia type) or a hyperacidity (Zollinger-Ellison syndrome type), and that PG I is a good indicator for gastric hypoacidity. Overlapping between normal and ulcer subjects is comparable to those obtained in acid output determinations.