Previous observations indicate that "CD5 B lymphocytes" are preferentially clustered in gut-related mesenchymal areas, such as peritoneum, thymus and tonsils. We have now found that pleuropericardial spaces contain an homogeneous population of large-sized, noncycling, nonsecretory B cells, expressing very high levels of surface IgM, little or no IgD, Mac-1 and low levels of B220. This phenotype and the over-representation of some antibody clonotypes suggest that the pleuropericardial cavity contains a pure "CD5 B cell" population. In all mouse strains analyzed, however, many of these cells are CD5-. These findings, together with the common origin of peritoneum and pleural layers in the primitive coelomic cavity, suggest that such B cells differentiate locally from intraembryonic precursors; we propose to designate them as "coelomic", to distinguish them from "stromal", bone marrow-derived B cells.