A mouse lymphocyte surface molecule which is selectively expressed by mucosal T cells and detected with the monoclonal antibody (mAb) M290 has provisionally been identified as a beta 7 integrin. Identification was based on close homology of its beta subunit at the N terminus with the recently reported, highly distinctive, human beta 7 sequence. mAb were prepared against the beta and alpha subunits of the mouse molecule, termed beta 7 and alpha M290, respectively, and used to study surface expression of the two components. beta 7 was present on most lymph node lymphocytes in association with alpha 4 rather than alpha M290. This integrin, beta 7 alpha 4, was shown to be identical to LPAM-1 (beta p alpha 4) the Peyer's patch homing receptor. Stimulation in vitro of mouse lymph node T cells with anti-CD3 in the presence of transforming growth factor (TGF)-beta increased beta 7 expression in about 40% of cells and changed the associated alpha chain from alpha 4 to the novel alpha M290 subunit, which, in most cells, was expressed de novo. Immunoprecipitation of beta 7 both from these cells and from intraepithelial lymphocytes gave closely similar results and showed predominance of the beta 7 alpha M290 integrin. It is suggested that in vivo this change in alpha-chain usage occurs in mucosal T cells in response to TGF-beta acting in the mucosal microenvironment and that the new integrin confers particular adhesive properties, possibly homing specificity, on the cells.