Cells separated from the wall of the umbilical cord vein by collagenase digestion could be identified as endothelial by their characteristic ultrastructure, their growth pattern in culture, and their microscopical morphology. These cells, both freshly explanted and after long-term culturing, were capable of stimulating allogeneic lymphocytes in vitro. Control experiments indicated that this stimulation was not attributable to contamination of the endothelial cell suspensions by foetal fibroblasts or passenger lymphocytes. The dose response characteristics and kinetics of the lymphoproliferative response using endothelial stimulating cells was similar to mixed lymphocyte cultures. Sera which were capable of inhibiting the mixed lymphocyte culture response were relatively ineffective in inhibiting the stimulation caused by endothelial cells.