The investigations to be performed include the determination of serum group II pepsinogen levels in controls, in patients with peptic ulcer disease and in patients with other disorders of the stomach, longitudinal studies of the serum group I pepsinogen response to histalog in patients having gastric surgery for peptic ulcer disease, the determination of group I and group II pepsinogen levels in gastric mucosal biopsies from patients with and without peptic ulcer disease, the purification of the individual components of the group I and group II pepsinogens, and the development of specific assays for the group I and group II pepsins.