Intraperitoneal injections of galactosamine-HCl in rats were followed by transitory elevations of serum alpha-fetoprotein (AFP) concentrations. These were associated with regeneration of the damaged liver; a maximum of serum AFP was reached on day 4. In sera of untreated rats, serum AFP levels were <0.1 µg/ml and no cellular AFP was detected in liver sections. Two days after galactosamine injections, AFP was localized for the first time in the cytoplasm of epithelial cells of bile ducts. The intensity of AFP staining reached a maximum between days 3 and 4. In addition, faint but distinct AFP-positive reactions were seen in the cytoplasm of randomly distributed hepatocytes. After day 5, AFP-staining cells rapidly disappeared. A strong correlation was noted between reappearance of AFP in sera, intensity of epithelial bile duct proliferation and cellular AFP staining.