Recent studies have documented the existence of at least two transmissible agents of posttransfusion non-A, non-B hepatitis. The disease frequently progresses to a state of chronic liver disease and/or persistent viremia, often in the absence of elevated liver enzymes. Difficulties encountered in the development of serologic tests for NANB antigens may be related to a lack of potent antibody coupled with low concentrations of infectious agent. The ultrastructural changes we have observed in infected chimpanzee hepatocytes are primarily confined to the cytoplasm and are, in the aggregate, most similar to those induced by a variety of RNA, but not DNA, viruses. At least one of the NANB hepatitis agents interferes with concurrent HAV or HBV infection in experimentally inoculated chimpanzees.