Electron microscopic observations were carried out on five HBsAg carrier chimpanzees infected with delta (a) agent and two chimpanzees infected with human non-A, non-B hepatitis. The cytoplasmic tubular structures, which have been recognized in the liver of chimpanzees infected with human non-A, non-B hepatitis, were found also in the liver of HBsAg carrier chimpanzees infected with 8 agent. The quantity of the cytoplasmic structures in serial studies was associated with SGPT elevation rather than with expression of 6 antigen in sera and liver tissues. This indicates that the cytoplasmic structures reflect a pathologic change of the hepatocytes in chimpanzees infected with 6 agent or human non-A, non-B hepatitis. These and other similarities between the two agents suggest a similar nature.The delta (6) antigen-antibody system was found to be invariably associated with hepatitis B virus (HBV); indeed, it was discovered by immunofluorescence in the liver cell nuclei of chronic hepatitis B surface antigen (HBsAg) carriers with liver disease (1). Electron microscopic studies did not reveal any consistent changes in the cytoplasm of hepatocytes from patients with d-associated hepatitis (2). In contrast, cytoplasmic tubular structures were seen in the livers of chimpanzees experimentally infected with non-A, non-B hepatitis agent but not with either hepatitis A or hepatitis B viruses (3, 4), and the cytoplasmic structures were considered to be characteristic markers of non-A, non-B hepatitis. The present study describes the finding of similar cytoplasmic structures in the liver of 6-infected chimpanzees.