Fibroblast growth factor (FGF) has been purified to homogeneity from human brain by a procedure involving salt precipitation, cation-exchange chromatography, Heparin-Sepharose affinity chromatography and reverse-phase HPLC. Isolation was monitored by radioimmunoassay and/or by testing column fractions for their capacity to stimulate the proliferation of vascular endothelial cells in vitro. The amino-terminal sequence of human brain FGF was determined as Pro-Ala-Leu-Pro-Glu-Asp-Gly-Gly-Ser-Gly-Ala-Phe-Pro-. This sequence is identical to that of the amino-terminal region of bovine FGF. Additional evidence, including amino acid composition, chromatographic retention behavior, and immunoreactivity suggest that the human and bovine mitogens are very similar in structure.