Idiotypes (Ids) are unique epitopes of Ab V regions and can trigger anti-Id immune responses, but immunization with several nonadjuvanted isologous IgG mAbs has induced tolerance to their Ids. We immunized non–lupus-prone mice with 11 allotype “a” of IgG2a (IgG2aa) and 4 IgG2c nonadjuvanted, isologous mAbs purified from serum-free medium. Of five IgG2aa mAbs with specificity for nucleosomes, the repeating histone-DNA subunit of chromatin, four elicited an IgG1 anti-mAb response and one mAb was nonimmunogenic. In contrast, none of six IgG2aa mAbs with unknown specificity triggered anti-mAb responses. The data suggested a link between immunogenicity and specificity for nucleosomes. One anti-nucleosome IgG2aa mAb, termed 3F7.A10, copurified with self-histones and was a potent immunogen for BALB/c mice. The response against IgG2aa 3F7.A10 was CD4+ Th cell–dependent, dominated by the IgG1 subclass, and Id specific. Ultracentrifugation converted the purified 3F7.A10 mAb into a weak immunogen, suggesting that the mAb had formed immunogenicity-enhancing immune complexes (ICs) with nucleosomal Ags during cell culture. BALB/c mice injected with viable MHC-incompatible 3F7.A10 hybridoma cells grown in serum-free medium mounted strong anti-Id responses. TLR9-deficient mice responded significantly weaker to Id-3F7.A10 than did TLR9-sufficient mice, suggesting that the cognate BCR efficiently internalizes the Id in an IC with nucleosomes. Passive transfer of IgG2aa 3F7.A10 to BALB/c mice with high titers of IgG1 anti-3F7.A10 led to glomerular deposits of IgG1/IgG2a complexes. The immunogenicity of Id-3F7.A10 raises the possibility that diverse Ids of nucleosome-specific Abs form ICs with nucleosomes released from dying cells and elicit spontaneous formation of anti-Id Abs in vivo.