The functional significance of MHC class II expression by vascular endothelial cells remains obscure. In this study the possibility that Ag presentation by endothelial cells (EC) influences T cell transmigration, facilitating the recruitment of Ag-specific T cells into tissues, was investigated. The frequencies of T cells with specificity for an HLA-DR alloantigen, or for the recall Ag tetanus toxoid (TT), were measured in peripheral blood CD45RO+(memory) CD4+ T cells before and after transmigration through γ-IFN-treated EC monolayers. Frequencies of anti-DR17, IL-2-secreting T cells were fourfold higher in the T cells that transmigrated through a monolayer of DR17-expressing EC. Similar increases were seen in TT-specific, DR7-restricted T cells that transmigrated through TT-pulsed, DR7-expressing EC. To examine more directly the effects of cognate recognition of Ag presented by EC, T cell clones were used. For clones that proliferated in a costimulation-independent manner to Ag presented by EC, cognate recognition arrested transmigration. In contrast, Ag presentation by EC to B7-dependent T cell clones, which do not proliferate following cognate recognition of EC, enhanced the rate of transendothelial migration. These data suggest that Ag presentation by EC may serve to augment the recruitment of Ag-specific T cells into tissues and that proliferation and transmigration are mutually exclusive T cell responses.