We found an endogenous growth factor, referred to here as heart-derived growth factor (HDGF), that stimulates the proliferation of vascular endothelial cells. HDGF was purified from bovine myocardium using a procedure that involves denaturation of undesired proteins with methanol and chloroform. Soluble HDGF was purified essentially to homogeneity in a single step by heparin affinity chromatography. The purified HDGF was identified to be acidic fibroblast growth factor based on the following properties: molecular weight of 18,000, isoelectric point of 5.2, amino acid composition and sequence, its dissociation from a heparin affinity column at 0.9 M NaCl, potentiation of activity in the presence of heparin, and antigenicity. Our yield of HDGF was 500 µg/kg of tissue. Antiserum raised to HDGF localized HDGF in the cardiac myocytes in culture. These data indicate that a large amount of acidic fibroblast growth factor is present in the heart, and the cardiac myocytes are likely to be a major source of it.