Two sera obtained from four healthy blood donors, which caused non-A, non-B post-transfusion hepatitis in two recipients, were experimentally inoculated into nine marmosets. Three of seven marmosets developed acute hepatitis characterized by the elevation of serum concentrations of glutamic pyruvic transaminase (GPT) and/or isocitric dehydrogenase (ICD) 8-11 weeks after inoculation. Four of seven showed histopathological changes of acute hepatitis in liver biopsy specimens during the biochemically acute phase. In electron microscopic examination, attached membrane-like structures, which consisted of two-unit membranes of two neighboring endoplasmic reticula with electron-dense material between them, were noted in cytoplasm of hepatocytes during the acute phase of hepatitis. Furthermore, acute-phase sera obtained from two animals were inoculated into four additional marmosets, and non-A, non-B hepatitis was successfully passaged in two of them. The results of this study indicate that certain species of marmoset monkeys are susceptible to human non-A, non-B hepatitis agents and provide a useful animal model for non-A, non-B hepatitis.