Under certain conditions, NK cells accumulate rapidly at extrahematic sites. In an effort to define the mechanisms underlying recruitment of NK cells in tissues, we investigated their ability to migrate across endothelial cell (EC) monolayers. A considerable proportion of NK cells adhered to EC and about 30 to 40% of the adherent NK cells migrated across EC. NK cells were two to three times more efficient than resting unseparated T cells but were considerably less efficient than monocytes. Exposure of NK cells to IL-2 or of EC to IL-1, TNF, or IFN-gamma augmented transendothelial migration. mAb directed against CD18 and CD11a inhibited binding and migration of NK cells across resting or IL-1-activated EC, whereas anti-CD11b and c mAb did not. Thus, the LFA-1 pathway is a crucial determinant of the adhesive and migratory interactions of NK cells with vascular endothelium. Using IL-1-activated EC, We found that anti-VLA-4 and anti-VCAM-1 mAb, utilized in concert with anti-CD18, significantly reduced adhesion and transmigration. The CS-1 peptide of fibronectin had no effect on binding and migration but, when used in concert with anti-CD18 and anti-VCAM-1 (but not anti-VLA-4), caused a small, but significant, increase in inhibition. The capacity to bind and migrate across endothelial monolayers underlies the recruitment of NK cells in tissues under certain physiologic and pathologic conditions.