A sulfated glycopeptide with antiulcer properties in animals and man was administered orally to 10 patients with duodenal ulcer, and its effects on peptic activity tested in vivo. A synthetic dipeptide,N-acetyl-l-phenylalanyl-l-diiodotyrosine, was used as substrate. Sulfated glycopeptide, 50 and 100 mg, completely inhibited peptic activity 15 minutes after administration in 2 of 3 patients and 3 of 3 patients examined, respectively; the inhibitory effect lasted 30 minutes. Sulfated glycopeptide, 200 mg, completely inhibited peptic activity in 4 of 4 patients examined 15 minutes after administration. In 2 patients complete enzymatic inhibition lasted 1 hour. In vitro, sulfated glycopeptide inhibited peptic activity of human gastric juice but not of purified human pepsin. These results may indicate that pepsin is present in gastric juice as a component of a mucoprotein complex. Sulfated glycopeptide may inhibit pepsin activity in gastric juice by directly precipitating this glycoproteic complex.