The rat monoclonal antibody (mAb) termed EA-1 was originally selected for its capacity to block the adhesion of T lymphocyte progenitors to mouse thymic endothelium. Here we show that the mAb EA-1 recognizes the alpha 6 chain of alpha 6 beta 1 and alpha 6 beta 4 integrins. Both molecules are present at a high level on the luminal and basolateral side of vascular endothelium and alpha 6 beta 1 integrin is expressed on the highly metastatic cell lines B16/129 (melanoma) and KLN-205 (carcinoma). These lung specific tumors bind preferentially to lung frozen sections, and EA-1 blocked this interaction in vitro. Moreover, mAb EA-1 inhibited experimental metastasis to the lung of B16/129 cells injected intravenously. Metastasis in vivo was blocked when the antibody was injected into mice before or simultaneously with the melanoma cells, as well as when melanoma cells were precoated with EA-1 before injection. We suggest that alpha 6 integrins play a dual role in the metastatic process, mediating the adhesion of tumor cells to the luminal surface of the endothelium and the adhesion to laminin in the subendothelial extracellular matrix during extravasation. Despite the fact that alpha 6 integrins are laminin receptors, EA-1 did not interfere with melanoma cell binding to laminin fragments. Our antibody EA-1 may therefore recognize a binding domain on alpha 6 integrins of a novel ligand involved in cell-cell interaction.