We investigated the chemotactic activity of interleukin (IL)-12 on human natural killer (NK) cells and other leukocyte subsets. It was found that IL-12 induced directional migration of highly enriched preparations of NK cells (> 80% CD16+ and CD56+) and CD3-activated T cells (both of CD4 and CD8 subset), but not resting T cells and monocytes. On the contrary, purified polymorphonuclear cells (PMN) showed significant and reproducible chemotactic response to IL-12. The effects of IL-12 on leukocyte migration were observed in a narrow concentration range with a peak at approximately 7.5 ng/mL, and were abrogated by monoclonal antibody (MoAb) anti-IL-12 or after cytokine boiling. We also investigated the interaction of NK cells with vascular endothelium in vitro. Overnight treatment of NK cells with IL-12 augmented their binding to cultured endothelial cells (EC) obtained from umbilical veins. IL-12-increased binding was better observed when resting rather than IL-1-activated EC were used as substratum of adhesion. IL-12-augmented binding of NK cells to resting or IL-1- activated EC involved the LFA-1/ICAM-1 and VLA-4/VCAM-1 pathways. Thus, by inducing migration and interaction with EC, IL-12 regulates crucial determinants of NK-cell recruitment in tissues.