The time course for appearance of antibodies to Borna disease virus (BDV) major antigens, p40, p24, p18 and p10 were investigated in BDV-inoculated adult rats by Western blotting. Anti-p10 antibodies were detected in sera as early as anti-p40 and -p24 antibodies at four or five weeks after inoculation. Furthermore, in addition to these major antigens of BDV, the rat serum could detect additional 80-, 58-, 43-, 20-, and 16-kDa proteins in BDV-infected cultured cells and/or animal brain cells by Western blot analysis. Of these proteins, the 20- and 16-kDa proteins were shown to be related to p24 protein by their reactivity with anti-p24 monoclonal antibody. Interestingly, the 58- and 24-kDa were found only in BDV-infected animal brain cells but not in cultured cells. The results in this study could provide a useful information on the mechanism for the viral replication and pathogenesis.