The identification of clonal human multipotent hematopoietic progenitors has permitted an analysis of the growth factor requirements for these cells. Human endothelial cell cultures were used to examine the effects of media conditioned by the endothelial cells on human multipotent (CFU-mix) and committed erythroid (BFU-E, CFU-E) and myeloid (CFU-GM) precursors. These studies demonstrate that endothelial cells produce proteins of approximately 30,000 daltons, with isoelectric focusing points of 4.5 and 7.2, which stimulate the growth of human BFU-E and CFU-mix. A heat-labile protein(s) of 30,000 and 15,000 daltons stimulated the proliferation and differentiation of granulocyte-macrophage (CFU-GM) colonies. No erythropoietin was detected in endothelial cell supernatants. This suggests that endothelial cells, a normal component of marrow stroma, play an active role in the modulation of human hematopoietic stem cell growth.