Endothelial cells were isolated from the hearts of neonatal mice and cocultured with syngeneic and allogeneic lymphocytes. T cells isolated by passage through nylon wool proliferated when cultured with allogeneic endothelium but not when cultured with syngeneic endothelium. This response was almost entirely confined to the CD8+ lymphocyte subset as purified CD4+ lymphocytes displayed a minimal response. Pretreatment of endothelial cells with recombinant murine gamma interferon induced expression of Ia but did not enable endothelial cells to activate CD4+ lymphocytes. Activation of CD8+ and T lymphocytes could be blocked with monoclonal anti class I antibody but was unaffected by anti-class II antibody. The failure to activate CD4+ lymphocytes was not due to suppression and did not lead to an anergic state. Instead, coculture of CD4+ cells with allogeneic endothelial cells induced a partial activation consisting of IL-2 receptor expression and accelerated secondary response kinetics.