Very-late antigen (VLA)-4(CD49d/CD29) constitutes the only member of the beta 1 integrin family that plays a role in the interaction of lymphoid cells with both extracellular matrix and endothelial cells through two identified ligands: fibronectin (FN) and VCAM-1, respectively. The expression and functional activity of VLA-4 has been studied in different maturation and activation stages of B cells from several cellular compartments. Resident B lymphocytes of different lymphoid organs were almost negative for VLA-4 as detected by both immunoperoxidase staining and flow cytometry analysis. However, a high expression of both chains of this heterodimer was observed when tonsillar B cells were activated in vitro with different stimuli, such as phorbol esters or Staphylococcus aureus Cowan I (SAC). Both nonactivated and in vitro activated B cells from peripheral blood constitutively expressed high levels of this surface antigen. The induced expression of VLA-4 after activation of tonsillar B lymphocytes was accompanied by the acquisition of the capacity to bind to a 38-kDa proteolytic fragment, containing the connecting segment I domain, of FN. Interestingly, nonactivated peripheral blood B cells were unable to attach to this FN fragment, in spite of their constitutive expression of VLA-4, and only acquired this functional capacity after cell activation with phorbol esters and SAC. This FN-binding acquisition was not affected by preincubation with inhibitors of protein and RNA synthesis. These results underline that the FN-binding activity of VLA-4 is dependent on processes affecting cellular activation as described for other members of the integrin family. By contrast, VLA-4-mediated homotypic aggregation of peripheral blood B cells could be triggered by anti-alpha 4 monoclonal antibodies independently of the cell activation state.