Collected samples serum of 30 MRL lpr/lpr and 15 MRL MP +/+ were tested by ELISA on purified mononucleosomes and its individual components, dsDNA and histones. Anti-mononucleosome antibodies can be detected as early as 10 and 23 week of age for MRL lpr/lpr and MP +/+, respectively, and arose before anti-dsDNA and anti-histone antibodies.