We have used monoclonal antibody binding, measured by radioimmunoassay, fluorescence flow cytometry, and ultrastructural immunocytochemistry, to measure expression of Ia antigens on cultured human umbilical vein endothelial (HUVE) cells. Under standard culture conditions, HUVE cells do not express Ia antigens. However, treatment of primary HUVE cultures with phytohemagglutinin induces the expression of Ia antigens. Every endothelial cell in the culture becomes Ia-positive and endothelial cells appear to synthesize Ia. HLA-A,B is concomitantly increased. The expression of Ia appears to be mediated by T cells because (a) pretreatment of primary HUVE cultures with OKT3 plus complement blocks the action of the lectins but not of medium conditioned by lectin-activated peripheral blood mononuclear cells; (b) co-culture of endothelial cells with allogeneic T cells, in the absence of lectin, also induces endothelial Ia; and (c) human immune (gamma) interferon, produced by Chinese hamster ovary cells transfected with the human gamma interferon gene, directly induces endothelial Ia. During co-culture with lymphocytes, about one-third of the endothelial cells are Ia-positive after 24 h and all of the endothelial cells are Ia-positive by 72 h. Proliferation of allogeneic T cells starts by 96 h and peaks at 144 h. Thus, endothelial Ia appears sufficiently early to be a determinant for the proliferation of allogeneic T cells. Inducible expression of Ia by endothelium may be important both for allograft rejection and for recruitment of circulating T cells into the site of an immune response.