The 48-1 antibody, initially reported to react specifically with non-A, non-B infected liver tissue, has been found to react also with liver specimens from chimpanzees infected with hepatitis delta virus (HDV). To clarify further the relation between HDV and appearance of the antigen reacting with the 48-1 antibody (48-1 Ag), immunoperoxidase studies were carried out on serial liver specimens from chimpanzees infected with HDV. Immunohistochemical and serological findings suggested that the appearance of 48-1 Ag paralleled that of HDV. Double immunoperoxidase staining revealed HDAg in the nucleus and 48-1 Ag in the cytoplasm of the same hepatocytes as well as in different hepatocytes separately. The course of appearance of microtubular aggregates paralleled that of 48-1 Ag. The present results suggested that expression of 48-1 Ag was related to infection with HDV, probably because expression of this antigen is induced from the host genome.