Galactosamine induces a dose-dependent hepatic injury in rats and many other animals. The toxicity of D-galactosamine appears to be a consequence of the loss of hepatic UTP. It has previously been reported that regenerating liver cytosol is able to prevent, at least in part, the lethal effect of this substance by stimulating hepatic regeneration. Recently, we have separated a fraction using alcohol precipitation (80%) from regenerating liver cytosol and from weanling rat liver cytosol prepared in acetate buffer (100 mM, pH 6.5). We named this fraction hepatic stimulatory substance because of its ability to stimulate DNA synthesis in vivo when injected intraperitoneally in 40% hepatectomized rats and in vitro in the presence of hepatocytes isolated and maintained in monolayer cultures. The stimulatory activity of the hepatic stimulatory substance is fully evident in subfractions of molecular weight up to 300,000 and 50,000 daltons of the crude material obtained using Amicon Ultra membrane filters. The present report describes the ability of hepatic stimulatory substance and its subfractions to stimulate hepatocyte proliferation and the application of these hepatic extracts in successfully reversing the lethality of D-galactosamine-induced hepatic necrosis in rats.(ABSTRACT TRUNCATED AT 250 WORDS)