We determined the kinetics and phenotypes of infiltrating B cells in gastric ulcer in the rat induced by gastric injection of acetic acid (day 0). Few B cells were found in the lesion in the early stages of ulceration. On day 40, two kinds of B cells with phenotypes of IgMhighIgDlowOX33 (CD45)+OX19 (CD5)- and IgMlowIgDhighOX33 (CD45)+OX19 (CD5)- were scattered in the granulation tissue of open ulcers, but not in the healed scar tissue. On day 180, those two kinds of B cells formed primary follicles in the granulation tissue of open ulcers, but were absent from the healed scar tissue. The IgMhighIgDlowOX33 (CD45)+OX19 (CD5)- B cell was considered to be identical to the marginal zone B cell of rat spleen. This phenotype of B cell might be associated with inflammatory process in chronic gastric ulcer.