A human hepatoma cell line synthesizes, as evidenced by metabolic labeling, an endothelial cell mitogen that is found to be mostly cell associated. The hepatoma-derived growth factor (HDGF) has been purified to homogeneity by a combination of Bio-Rex 70, heparin-Sepharose, and reverse-phase chromatography; it is a cationic polypeptide with a molecular weight of about 18,500-19,000. HDGF is structurally related to basic fibroblast growth factor (FGF). Immunological analysis demonstrates that antiserum prepared against a synthetic peptide corresponding to the amino-terminal sequence of basic FGF cross-reacts with HDGF when analyzed by electrophoretic blotting and by immunoprecipitation. Sequence analysis of tryptic fragments demonstrates that HDGF contains sequences that are homologous to both amino-terminal and carboxyl-terminal sequences of basic FGF.