A growth factor and two growth inhibitors for adult rat hepatocytes were purified from rat platelets. Hepatocyte growth factor (HGF) seemed to be a hepatotropic factor which has not been purified during the past 30 years. One of growth inhibitors was identical to TGF-β. The other, named PDGI-α, was a new growth inhibitor. TGF-β masking protein that may be a regulator of the activity of TGF-β was also isolated from rat and human platelets.