Conditioned media from cultured Kupffer and mononuclear macrophagic cells obtained 48 hr after CCl4 administration to rats contains chemotactic factors for human skin fibroblasts and human monocytes. The chemotactic mediator for fibroblasts was approximately 17 kD and was more prominent at early stages of culture. It induced a dose-dependent chemotactic response in fibroblasts. Although the conditioned medium from cultured Kupffer cells of normal rats also contained detectable biological activity, it was significantly less than that in conditioned medium from cultured Kupffer cells from CCl4-treated rats. The activity obtained after purification by highperformance liquid chromatography was completely ablated by incubation with tumor necrosis factor-α antibody. Transforming growth factor-β antibody diminished biological activity by 20%. Human recombinant tumor necrosis factor-α and transforming growth factor-β used in the assay as control showed significant chemotactic activity. The chemotactic activity present in whole normal conditioned medium was only present after 24 and 48 hr of culture. Furthermore, this activity was not neutralized by human recombinant tumor necrosis factor-α or transforming growth factor-β antibodies. Incubation of whole 6-hr conditioned medium with human recombinant tumor necrosis factor-α and transforming growth factor-β antibodies demonstrated and confirmed that tumor necrosis factor-α plays a major role in inducing the chemotactic response. On acidification of this supernatant, we found a notable increase in the biological response that could be neutralized by transforming growth factor-β antibody. Thus tumor necrosis factor-α and transforming growth factor-β may sequentially provide important signals for fibroblast and monocyte recruitment in vivo at initial stages of liver injury. (HEPATOLOGY 1991;14:895–900).