Sulfates of oxidized starch and its reduced products were investigated for their protective effect on experimental peptic ulceration. 1) Both oxidized starch sulfate and sulfate of reduced product of oxidized starch with various sulfur content and viscosity showed protective effect on ulceration of Shay rats when administered in 30 mg. doses intra-esophageally, and the protective effect is enhanced with increasing sulfur content, just the same as their inhibitory activity on proteolytic action in vitro. 2) Their protective effect on ulceration is closely related to the loss of pepsin activity in gastric juice. The decrease in pepsin activity is proportional to the decrease in ulceration rate. From the fact that they had no protective effect on ulceration by subcutaneous administration, their protective effect may be caused by their direct inhibition of pepsin activity in gastric juice. 3) They had no protective effect on Takagi's stress rats by intra-esophageal administration.