Protective effect of lipopolysaccharide (LPS) from various sources on gastric ulcer has been examined in mice using parenteral as well as oral route. Ulcer is induced by indomethacin, stress or alcohol. LPS was prepared from 6 species of bacteria (Escherichia coli, Pantoea agglomerans, Serratia ficaria, Enterobacter cloacae, Bordetella pertussis, Alcaligenes faecalis) and from wheat flour. When administered intravenously, LPS of Pantoea agglomerans was the most effective among other LPS examined. Lipopolysaccharide of wheat flour (LPSw) showed a significantly protective effect by the oral route, especially when given ad libitum in drinking water to mice.