We have generated a monoclonal antibody (mAb), 9EG7, against mouse endothelial cells that blocks adhesion of lymphocytes to endothelial cells. Sequencing of four tryptic peptides of the purified antigen revealed its identity with the integrin chain beta 1. The only beta 1 integrin that is known to mediate cell-cell adhesion is alpha 4 beta 1 (VLA-4). This is not the integrin that is functionally defined by the mAb 9EG7 on endothelial cells. First, alpha 4 is not present on the analyzed endothelial cells. Second, mAb 9EG7 does not block the cell-adhesion function of alpha 4 beta 1 on the nonactivated mouse lymphoma L1-2. Thus, the mAb 9EG7 can functionally distinguish between different beta 1 integrins and defines a beta 1 integrin other than alpha 4 beta 1 as a newly discovered cell-cell adhesion molecule. This integrin is most likely alpha 6 beta 1, since an antibody against the alpha 6 chain blocks lymphocyte adhesion to the same degree as the mAb 9EG7, the effect of both antibodies is not additive, and the alpha 6 chain is coprecipitated with beta 1 in 9EG7 immunoprecipitations. Surprisingly, activation of alpha 4 beta 1 on L1-2 cells with phorbol ester or Mn2+ allows blocking of alpha 4 beta 1-mediated adhesion of L1-2 cells to endothelial cells with mAb 9EG7. Furthermore, only the activated alpha 4 beta 1 heterodimer, but not the unactivated complex, is detectable with 9EG7 in immunoprecipitations and by flow cytometry. Thus, mAb 9EG7 defines an epitope on integrin chain beta 1, which is accessible on the alpha 4 beta 1 heterodimer only after activation of this integrin.