Two chimpanzees with persistent non-A, non-B (NANB) hepatitis were superinfected with marmoset-passaged MS-1 HAV. Two control chimpanzees were also infected with marmoset-passaged HAV. Neither animal with persistent NANB hepatitis developed elevated alanine aminotransferase (ALT) activity, whereas both control chimpanzees exhibited ALT elevations within 3 weeks after inoculation. In addition, both NANB-infected chimpanzees demonstrated a delayed anti-HAV antibody response in which one animal failed to produce detectable IgM anti-HAV. With the exception of one stool, all serial liver biopsy specimens and daily stool suspensions from the superinfected chimpanzees were negative for HAV antigen. One chimpanzee with a chronic HBV infection was superinfected with non-A, non-B hepatitis and was shown to develop elevated ALT activity and hepatocyte ultrastructural alterations accompanied by a marked reduction in the titer of serum HBsAg. Our combined findings indicate that acute and persistent non-A, non-B hepatitis infections are capable of interferring with two distinctly different hepatotropic viruses. These results also suggest that in vitro detection of non-A, non-B hepatitis infection or virus(es) may be achieved by antibody-independent methodologies that employ the basic principle of viral interference.