In this study, we demonstrated that immobilized fibronectin (FN) enhanced LAK activity, and that the enhanced LAK activity was completely abrogated by an anti-VLA-5 monoclonal antibody and RGD peptide. Fresh -spleen cells expressed VLA-4, VLA-6 and vitronectin receptor, whereas VLA-5 was expressed only on the spleen cells activated with IL-2. LAK cells showed increased adhesion to immobilized FN compared with that to control BSA, and the increased adhesion of LAK cells to immobilized FN was inhibited by anti-VLA-5 monoclonal antibody. Conjugate-formation assay showed that the LAK cells cultured on immobilized FN bound to target cells more efficiently than the control LAK cells, and that anti-LFA-1 monoclonal antibody inhibited the LAK-target cell binding. Immobilized type IV collagen and laminin, as well as FN, enhanced LAK activity. All these results suggest that the interaction of inte-grins expressed on LAK cells with extracellular matrix proteins act as co-stimulator for the enhancement of LAK activity , and that anchorage is necessary for full activation of LAK cells.