The effects of interferons (IFNs) on lymphocyte adhesion to cultured human vascular endothelial cells (EC) were investigated using an in vitro assay. Endothelial cells obtained from umbilical vein were first cultured at a low density with a conditioned medium (CM) from 12-O-tetra decanoylphorbol 13-acetate-concanavalin A (TPA-Con A) stimulated human peripheral blood lymphocytes (PBL), or with recombinant (r) gamma interferon (IFN-gamma) or r alpha interferon (IFN-alpha), and then were incubated with freshly isolated PBL. Natural IFN-gamma in the TPA-Con A CM and rIFN-gamma (12.5-500 U/ml) induced major histocompatibility complex-class II antigens (HLA-DR, HLA-DP, and HLA-DQ) and significant lymphocyte adhesion to the EC, whereas rIFN-alpha did not. The lymphocyte adhesion to the EC and the expression of DR antigens on the EC were well correlated in terms of both kinetics and the dose-response pattern of rIFN-gamma. When EC expressing I region associated (Ia) antigen were preincubated with monoclonal anti-DR antibody before the addition of lymphocytes, the lymphocyte adhesion was significantly inhibited in both allogeneic and syngeneic combinations, whereas anti-HLA-DP, anti-HLA-DQ, and anti-HLA-ABC antibodies did not inhibit the binding at all. Cell fractionation experiments indicated that the majority of lymphocytes adhering to Ia-expressed EC were Leu-3+ T cells, whose binding was again almost completely inhibited by anti-DR antibody. Moreover, anti-Leu-3a, but not anti-Leu-2a, antibody effectively inhibited the T cell adhesion to the EC. These results strongly suggest that the interaction of the Leu-3(T4) receptor of T cells with IFN-gamma-induced DR antigens on EC plays a central role in the selective adhesion of Leu-3+ T cell to EC.