Two forms of acidic fibroblast growth factor were isolated from bovine brain by a combination of ammonium sulfate precipitation, cation-exchange chromatography, heparin-Sepharose affinity chromatography, and reverse-phase high-performance liquid chromatography. Amino acid analysis, polyacrylamide gel electrophoresis and amino-terminal sequence analysis showed that one form corresponds to a protein with a molecular mass of 16 kDa and the amino-terminal sequence Phe-Asn-Leu-Pro-Leu-Gly-Asn-Tyr-Lys-Lys-Pro-Lys-Leu-Leu-Tyr- and thus represents the acidic fibroblast growth factor as previously characterized [Böhlen et al. (1985) EMBO J. 5, 1951-1956]. The second mitogen form has a molecular mass of 15.5 kDa. The amino-terminal sequence was established as Asn-Tyr-Lys-Lys-Pro-Lys-Leu-Leu-Tyr-. This evidence indicates that the latter form represents an amino-terminally truncated acidic fibroblast growth factor, lacking the first six amino acid residues. Both forms of the protein are biologically active and equipotent with respect to stimulation of the proliferation in vitro of mesodermal cells such as vascular endothelial and adrenal cortex cells.