The reactivity of a monoclonal antibody, ER-TR9, that demonstrates heterogeneity among mononuclear phagocytes is described. In the spleen, ER-TR9 exclusively reacts with a population of macrophages located in the marginal zone. ER-TR9 does not react with macrophage antigen 1-positive red pulp macrophages or any other types of splenic stromal cells. ER-TR9+ ve cells localize in anatomical proximity of a subpopulation of B cells, i.e., B cells that are immunoglobulin M positive and weakly positive to negative for immunoglobulin D. The possible significance of this particular interaction between both cell types during the immune response is discussed.