Some species of marmosets are susceptible, not only by parenteral inoculation but also by oral exposure, to human hepatitis A virus present in sera or feces. The stools of animals inoculated parenterally or orally contained fecal antigen during certain times of the incubation period and the early, acute phase of the disease; viruslike particles were present in feces of orally infected animals and such feces were infectious when inoculated into marmosets. The fecal antigen crossreacted both with the fecal virus particles and the immune-adherence antigen (see also papers by Purcell et al and Hilleman et al). The MS-1 and CR-326 strains of hepatitis A appeared antigenically similar or identical whereas the GB strain was antigenically different and may be associated with the recently defined type of hepatitis termed hepatitis C or hepatitis non-A/non-B. On repeated challenge hyperegic responses with diffuse liver cell necrosis occurred in some immune animals and this phenomenon must be taken into account in any future vaccination studies.