Natural hepatocyte growth factor (nHGF) purified from human serum showed a difference in molecular mass (Mr) between SDS PAGE (76-90 kDa) and gel filtration chromatography on a Sephadex G-200 column (> 200 kDa), whereas nHGF or recombinant HGF (rHGF) from cell culture medium did almost the same Mr (74-100 kDa). A bound form of rHGF with heparan sulfate (or heparin), and an aggregate form of rHGF itself showed a homogeneous band with a Mr of 76-90 kDa on SDS PAGE, but showed a Mr larger than 200 kDa on a Sephadex G-200 column. Both nHGFs, rHGF and the bound form were basic, but the aggregate form was acidic in ionic nature. No significant difference was found in affinity for heparin among these HGF preparations. The bound form treated by the procedures for purification of nHGF from human serum still showed a larger molecular form. The bound form mimicked physicochemical properties of nHGF purified from human serum. These results suggest that a possible form of nHGF in human serum may be a bound form with heparin-like molecules such as heparan sulfate, which are found in the circulation and on cell surface, and purified as the bound form.