Rabbits infected with the L strain of rinderpest virus (RV) produced high titres of antinuclear antibodies (ANA) which reached a maximum two weeks after inoculation but rapidly disappeared by 6-8 weeks. These ANAs reacted with HeLa cells by indirect immunofluorescence test resulting in a homogeneous nuclear fluorescence. In order to investigate the target antigens of ANAs, the effects on the nuclear fluorescence pattern of pretreating HeLa cells were examined: DNase 1 treatment resulted in a decrease in the fluorescence whereas no changes were evident after RNase A treatment. Some group of sera showed decreased fluorescence in the cells from which histones were acid extracted, but other groups did not change in fluorescence. Sera which had failed to react with acid extracted cells gave positive fluorescence following histone reconstitution. The results indicate that DNA and nucleohistone are the major target antigens for ANAs. In addition, antibodies against nucleoli and extractable nuclear antigens were induced in some rabbits.