Human vascular cells are capable of stimulating granulopoiesis in agar culture of human bone marrow cells. This effect was obtained by including vein fragments in the culture or by using endothelial cells separated from the vein of human umbilical cords as feeder cells. Furthermore, the stimulatory capacity of conditioned medium obtained from cord veins was found to be highly active in comparison to that obtained from peripheral leukocytes. Endothelial cells within the bond marrow cavity are suggested as a local source of factors regulating granulopoiesis in humans in addition to the monocyte.