Extended amino terminal sequence determinations, made on both acidic and basic fibroblast growth factors from human brain, showed extensive homology with each other and with their respective bovine counterparts. Both human growth factors in the presence of heparin have equivalent specific mitogenic activities on human umbilical vein endothelial cells in culture whereas in the absence of heparin, the acidic mitogen is less than 1% as active as the basic growth factor.