We have identified long-incubation viral agents that share epitopes with hepatitis B virus (HBV). During chimpanzee infectivity studies, these agents may be recognized in the liver since they possess complementary nucleic acid sequences with HBV DNA; the genomic size was found to be 3.2 kilobases, identical to that of HBV. Liver injury was produced and there was antigen expression in hepatocytes. Chimpanzees were not protected by prior immunization with hepatitis B surface antigen; conversely, they were still susceptible to HBV after recovery from infection with such agents. These findings suggest that these hepatitis B virus-related variants appear to be immunologically distinct from HBV.