Serum pepsin activity was measured in 359 patients with various gastric diseases, the serum protein themselves serving as substrate. In 332 out of 359 patients, BAO and PAO, and in 173 of these patients, pepsin activity, were determined in gastric juices. In 102 patients, serum pepsin activity was measured before, and 45 and 90 minutes after, pentagastrin stimulation. In 71 out of these 102 patients, pepsinogen I was measured simultaneously by RIA. There was good correlation between stimulated serum pepsin activity and maximal pepsin output (r = 0.63), as well as BAO (r = 0.67) and PAO (r = 0.77). The higher the PAO, the greater the increase in pepsin activity in the sera after pentagastrin stimulation. The increase in pepsinogen I under stimulation was smaller in the sera than pepsin activity. There was a slightly better correlation of PAO with stimulated serum pepsin activity than with pepsinogen I (r = 0.76 versus 0.62). The measurement of pentagastrin-stimulated pepsin activity in sera, as a very simple and inexpensive method, permits the screening, with an overall accuracy of more than 75%, of achlorhydrics (stimulated pepsin activity less than 30 micrograms thyrosine/24 hours/ml serum), acid secretors (greater than 80 micrograms thyrosine/24 hours/ml serum) and hypersecretors (PAO greater than 35 mmol HCl/hours, greater than 120 micrograms thyrosine/24 hours/ml serum).