CD46, membrane cofactor protein, is a glycoprotein widely present on the cell membranes and is different in molecular weight among organs or cells in the same individual. It acts as a cofactor of I of the complement system, which inactivates C3b bound to the autologous cells, and protects them from the attack of the complement. In this study CD46 was demonstrated in the stomach immunochemically. Gastric CD46 was expressed strongly in the mucosal epithelium, mucosa and endothelial cells of the vessels in the submucosal layer, whereas expressed weakly in the submucosa and muscle. Western blot analyses revealed that gastric CD46 was expressed as one broad band with molecular weight ranging from 60 kDa to 69 kDa, which was distinct from that of lymphocytes in the peripheral blood.