Interactions between homing receptors on circulating leucocytes and endothelial addressins regulate tissue-specific cellular extravasation. Although integrin alpha 4 beta 7 appears to be the main receptor for gut-homing T lymphocytes, less is known about molecules mediating mucosal B cell homing. Expression of integrin alpha 4 beta 7 on B lymphocytes, B cell blasts, and plasma cells in human gut-associated lymphoid tissue (GALT; the Peyer's patches and appendix) and lamina propria was studied by multi-colour immunofluorescence applied on cryosections. Isolated mononuclear cells from the same tissue compartments were examined by flow cytometry and compared with peripheral blood B cells. Integrin alpha 4 beta 7 was expressed by IgA+ B cell blasts and plasma cells (CD38high) in the lamina propria, B cell blasts in GALT, and sIgD+ B lymphocytes in peripheral blood. In contrast, GALT sIgD+ B lymphocytes were negative or only weakly positive for alpha 4 beta 7. These results suggested that B lymphocytes down-regulate alpha 4 beta 7 upon extravasation in GALT but up-regulate this integrin after antigen-priming. Thus, alpha 4 beta 7 may be a homing receptor also for B cell blasts extravasating in the gut lamina propria, where this integrin is maintained on plasma cells, perhaps as a local retention factor.