The presence of heparin-binding growth factors in liver was investigated by measuring the DNA synthesis stimulatory activity of liver extracts using quiescent fibroblasts as target cells. It was found that cytosolic fractions of mouse, rat and human liver, as well as isolated rat hepatocytes, contain a large amount of growth stimulatory activity. Most liver cytosolic activity is due to heparin-binding growth factors, because > 90% of the activity bound to a heparin affinity column in the presence of 0.8 mol/L NaCl, and was quantitatively eluted with 2 mol/L NaCl. Purification of these factors from both mouse and rat liver indicated the presence of both heparin-binding growth factor-1 and 2 in liver extracts. The level of the heparin-binding growth factors, as estimated from the biological activity, is approximately 1 μg/gm mouse liver and 0.1 μg/gm rat and human liver. Heparin-binding growth factor-1–like factors were 10 times as abundant as heparin-binding growth factor-2–like factors. These data indicate that the cytosolic fractions of mouse, rat and human liver contain heparin-binding growth factors as the primary growth factor for fibroblasts, and heparin binding growth factor-1–like molecules account for most of the cytosolic activity in both mouse and rat liver. (HEPATOLOGY 1991;13:6–14).