Abstract To investigate the mechanism of autoantibody production, the extent and nature of anti-DNA precursors within normal human B cells were examined by utilizing two different polyclonal B cell stimulators, Staphylococcus aureus Cowan I (SA) and immobilized mAb to the CD3 molecular complex (64.1). In cultures stimulated with SA and IL2, B cells produced IgM-RF, but not anti-DNA, whereas B cells produced great amounts of anti-DNA in cultures stimulated with SA and intact T4 cells. In cultures stimulated with immobilized anti-CD3, T4 cells that had been treated with mitomycin C (T4 mito) induced the production of anti-DNA as effectively as that of IgMRF. Limiting dilution analyses revealed that the precursor frequency of anti-DNA-producing cells in SA-stimulated cultures was markedly increased in the presence of intact T4 cells (0.399 to 2.549 per 10 4 B cell) compared with that in the presence of factors generated from mitogen-activated T cells (TF) (0.022 to 0.151 per 10 4 B cells). Thus, the proportion of IgM-secreting cells that produced anti-DNA in cultures with SA + T4 cells (12 to 31%) was much greater than that noted in cultures with SA + TF (3.4 to 4.0%), whereas the proportion of IgM-secreting cells that produced IgM-RF was not significantly different in these cultures (17 to 50%). The precursor frequency of anti-DNA-producing cells (0.019-0.097 per 10 2 B cells) was almost the same as that of IgM-RF producing cells (0.025-0.104 per 10 2 B cells) in cultures stimulated with immobilized anti-CD3. Of note, addition of SA increased the precursor frequency of IgM-RF-producing cells, but not that of anti-DNA-producing cells, in anti-CD3-stimulated cultures. These results indicate that T cells, but not T cell-derived cytokines, play a central role for the production of anti-DNA antibodies. Moreover, the data support the conclusion that the precursors of anti-DNA-producing cells have activation requirement different from those of IgM-RF producing cells.