We have observed the development of long-term sequelae in four cases of experimentally induced non-A, non-B (NANB) hepatitis in chimpanzees. These sequelae were characterized by the following manifestations: nonprotection against challenge with autologous infectious plasma following acute disease and subtle histopathological alterations typical of long-lasting viral hepatitis. These manifestations were observed in animals infected with either of two human inocula. Whether or not these inocula represent sources of single or multiple etiologic agents is not known. However, our studies suggest that these inocula share at least one common etiologic agent. Further, these results may represent an atypical chronology of convalescence from viral hepatitis infection. For example, the convalescent stage of a type B hepatitis infection may be expected to occur within 6 to 8 months following exposure, whereas true convalescence in NANB hepatitis may be protracted over several months to several years. Thus, future efforts to identify the causative agent(s) of NANB hepatitis, and efforts to define the immune response in NANB, must take into consideration these studies.