B lymphocytes express several adhesion molecules that are involved in cell-cell and cell-extracellular matrix interactions. The alpha 4 beta 1 integrin VLA-4, expressed on pre-B and mature/activated B cells, mediates adhesion of these cells to its two ligands, VCAM-1 and fibronectin. Recent evidence suggests that VLA-4 is involved in T lymphocyte activation; however, relatively little is known of the role of VLA-4 in B cell differentiation. To begin to assess the potential involvement of VLA-4 in B cell activation, we have examined the effect of ligation of VLA-4 on protein tyrosine phosphorylation in B cells. We found that cross-linking of VLA-4 by either mAb or natural ligands (i.e., VCAM-1 and the FN-40 cleavage fragment of fibronectin) induced the tyrosine phosphorylation of a 110-kDa protein in a human pre-B cell line (Nalm-6), an EBV-transformed B cell line (SB), and normal tonsillar B cells. These findings suggest that VLA-4 can activate a tyrosine kinase in B cells and B cell lines. These signals may be involved in the subsequent differentiation of pre-B and mature B cells within specific microenvironments where VLA-4 mediated adhesion is operational.