Endothelial cells, separated from the inside of the umbilical vein by collagenase digestion, could clearly stimulate allogeneic lymphocytes to blastogenesis and increased DNA synthesis in mixed lymphocyte endothelial-cell cultures. Many of the characteristics of these mixed cultures were similar to those found in mixed cultures consisting exclusively of allogeneic lymphoid cells; the MLC response. Endothelial cells could also be destroyed in vitro by cells sensitized in mixed lymphocyte cultures. When incubated in the presence of specific HL-A antisera, they were destroyed in non-immune peripheral blood mononuclear cells. These three reactions are thought to be in vitro correlated to some of the events taking place following allografting in vivo.