In vitro studies have shown hepatocyte growth factor (HGF) to be a potent mitogen for hepatocytes. Direct evidence of a mitogenic role in vivo was sought by inhibiting HGF activity, using continuous administration of neutralizing antibody to rats which had a stimulus for liver regeneration. Alzet osmotic mini-pumps, administering a constant supply of anti-HGF monoclonal antibody (clone D9), were inserted intraperitoneally into male Wistar rats; an irrelevant isotypical antibody was administered to controls. Forty-five animals received an intragastric bolus of 40 per cent carbon tetrachloride (CCl4) and groups of three test and control animals were killed at 24 h intervals for 7 days. Treatment with anti-HGF monoclonal antibody significantly inhibited the levels of immunodetectable HGF in the sera of rats following CCl4 administration. In comparison with controls, hepatocyte proliferation as assessed by bromodeoxyuridine labelling in anti-HGF-treated animals was significantly inhibited at 24 h (P < 0.001), 48 h (P < 0.001), and 96 h (P < 0.05) post-CCl4 administration. In contrast, sinusoidal cell proliferation was not significantly different from controls at any time point. Inhibition of the parenchymal proliferative response to acute CCl4-induced liver injury by the in vivo neutralization of HGF provides direct evidence that this growth factor plays an important role in liver regeneration following necrosis.