Previous studies in rats and dogs have implicated the liver as the source of some of the factors which are responsible for its regeneration. In the present study extracts from regenerating and normal livers were infused into the portal vein of pigs after a 15% partial hepatectomy. Thymidine kinase activity was measured on the 3rd and 4th days as an index of liver regeneration. An intraportal infusion of extracts from regenerating, but not normal, livers resulted in an increased regenerative response. The response was dose-related. The stimulator substance was stable when stored at -20 degrees C and could be precipitated by absolute ethanol.