We have previously reported that peptide 88-99 of histone H4 represents a minimal T cell epitope recognized by Th cells from nonautoimmune BALB/c (H-2d/d) mice immunized with nucleosomes. In this study, we tested a panel of overlapping peptides spanning the whole sequences of H4 and H3 for recognition by CD4+ T cells from unprimed (New Zealand Black (NZB) × New Zealand White (NZW))F1 lupus mice (H-2d/z). None of the 11 H4 peptides was recognized by CD4+ T cells from (NZB × NZW)F1 mice. In contrast, these cells proliferated and secreted IL-2, IL-10, and IFN-γ upon ex vivo stimulation with H3 peptides representing sequences 53-70, 64-78, and 68-85. Peptides 56-73 and 61-78 induced the production of IFN-γ and IL-10, respectively, without detectable proliferation, suggesting that they may act as partial agonist of the TCR. Th cells from unprimed BALB/c mice and other lupus-prone mice such as SNF1 (H-2d/q) and MRL/lpr (H-2k/k) mice did not recognize any peptides present within the H3 region 53-85. We further demonstrated that immunization of normal BALB/c mice with syngeneic liver nucleosomes and spleen apoptotic cells, but not with nonapoptotic syngeneic cells, induced Th cell responses against several peptides of the H3 region 53-85. Moreover, we found that this conserved region of H3, which is accessible at the surface of nucleosomes, is targeted by Abs from (NZB × NZW)F1 mice and lupus patients, and contains motifs recognized by several distinct HLA-DR molecules. It might thus be important in the self-tolerance breakdown in lupus.