B cells in normal human Peyer's patches and in primary B cell lymphomas of the stomach have been characterized in terms of their cellular morphology and their reactivity with a panel of monoclonal antibodies. A population of B cells is present in normal and malignant gut-associated lymphoid tissue which is composed of neither mantle zone cells nor germinal centre cells. In Peyer's patches these cells surround the follicles merging with the mantle zone and extending both into the dome region, infiltrating between the epithelial cells and also towards the serosa. They are intermediate in size with irregular nuclear outlines and they resemble the centrocytes in the follicle centre. They are quiescent, expressing C3b- and C3d-receptors and surface IgM but not surface IgD. These centrocytes-like cells which are not seen in the peripheral lymph nodes are identical to the B cells in the marginal zone of the spleen according to all of the criteria employed in this study.