Endothelial cell proliferation is a histologic characteristic of several forms of nephritis characterized by infiltration of the glomerulus with mononuclear cells. To investigate the mechanism mediating this event, human endothelial cells isolated from umbilical veins and cultured in vitro were incubated with supernatants of cultured human mononuclear cells. Supernatants from mononuclear cells exerted a dose-dependent stimulatory effect on endothelial cell proliferation. The stimulatory effect of supernatant was almost entirely removed by prior depletion of mononuclear cells of monocytes by adherence, suggesting that a monocyte product was responsible for the activity. To investigate the nature of the ligand responsible, partially purified human interleukin I added to endothelial cell cultures was found to stimulate cellular proliferation.