We investigated the influence of IL-4 on the interaction between Natural Killer (NK) cells and vascular endothelial cells (EC). Pretreatment of NK cells with IL-4 inhibited the adhesion of NK cells on resting or IL-1-activated EC. The inhibitory action of IL-4 was observed on both unstimulated NK cells as well as on cells concomitantly activated with IL-2 or with phorbol ester. IL-4 also inhibited the cytotoxicity of IL-2 activated NK cells on EC. Binding of NK cells to vascular EC involves the LFA1/ICAM-1 and 2, and VLA-4/VCAM-1 adhesion pathway. Anti-CD18 mAb had a lower inhibitory effect in IL-4-treated NK cells compared to control. The levels of inhibition with resting vs IL-1-activated EC, as well as antibody blocking experiments, suggested that IL-4 exerts an inhibitory influence predominantly on the beta 2-integrin (CD18)-dependent adhesion pathway; nevertheless IL-4 did not affect the expression of CD18/CD11a and VLA-4 on the NK cell surface, as assessed by flow cytometry. NK cells are potent producers of IFN-gamma and there is evidence that they play a role in orienting immunity to the TH1-type responses. The inhibition by IL-4 of NK cell binding to EC and subsequent recruitment and activation may thus contribute to development of TH2 responses.