We compared B-cell phenotypes in Peyer's patches and solitary lymphoid follicles (organized gut-associated lymphoid tissue, GALT) with those in jejunal or ileal lamina propria. In situ, immunostaining showed that small B cells of naive [surface immunoglobulin D-positive (sIgD+) CD27-] and memory (sIgD+/- CD27+) phenotypes occurred almost exclusively in GALT, whereas the lamina propria contained only scattered sIgA+ CD27+ memory cells. In contrast, B-cell blasts and plasma cells negative for CD20 and often also for CD19 but with strong expression of CD38, CD27 and cytoplasmic IgA (cIgA), dominated in the lamina propria but were scarce in GALT. By flow cytometry, the proportion of dispersed CD19+ B lymphocytes varied from 4 to 42% among jejunal mucosal samples; between 5 and 50% of these were sIgD+, suggesting a variable contamination with GALT cells. B-cell blasts and plasma cells, identified by their large size and strong expression of CD38, were regularly found (25-35% of the total mononuclear cell population). Distinction between B-cell blasts and mature plasma cells was made by the presence or absence of human leucocyte antigen (HLA) class II molecules, CD45RA, CD19 and surface immunoglobulin. No CD19+ B cells outside GALT expressed CD5, but a very small portion of the lamina propria B-cell blasts were positive for CD28. Dispersed sIgA+ lamina propria cells expressed low levels of CD40, proliferated on CD40 ligation and constitutively secreted IgA in vitro. We concluded that the lamina propria B-cell compartment consists mainly of B-cell blasts and plasma cells but also has scattered, small sIgA+ cells that can proliferate in response to CD40 ligation and may therefore function as local memory cells for recall antigens.