Samples of serum, feces and liver tissue and organs of six cotton-eared marmosets Callithrix jacchus infected intravenously with two different strains of hepatitis A virus (HAV), were studied by conventional histologic techniques, by serological techniques and by immunocytochemical methods, such as immunofluorescence (IF) and peroxidase-antibody techniques. Hepatitis A antigen (HAAg) was detectable in daily collected stools, in liver biopsy obtained sequentially, and in organs collected at necropsy. Two marmosets also developed antibodies to HAV. By contrast, serum transaminases were not altered and there were histological hepatic lesions consistent with acute viral hepatitis in all inoculated animals. The data obtained, demonstrate that these primates are susceptible to human HAV and may be a useful animal model for the study of infection by this virus.