BALB/c mice rendered tolerant by the neonatal injection of semiallogeneic (C57BL/6 X BALB/c)F1 spleen cells develop features of autoimmune disease. The possible mechanisms involved in autoantibody production, particularly anti-DNA antibodies, were investigated. In the first 5 wk, there was polyclonal B cell activation, as indicated by marked hypergammaglobulinemia, with a predominance of IgG1 and an increased production of antihapten antibodies. IgG1 anti-SSDNA and anti-DSDNA antibodies were detected with similar kinetics, but at higher titers than the anti-hapten antibodies. Also, there was a correlation between the effective induction of tolerance, as evaluated by the measurement of alloantigen-specific cytolytic T lymphocyte precursors, the persistence of B cell chimerism, and the production of anti-DNA antibodies. Anti-DNA antibodies were observed only in mice exhibiting a persistence of immunoglobulins bearing the donor's allotype. To determine the origin of anti-DNA antibodies, experiments were conducted whereby newborn BALB/c (Igh-1a) mice were injected with F1 cells from mice resulting from a crossing between Igh congenic BALB/c mice bearing the IgCHb allotype and conventional C57BL/6 mice (Igh-1b). All anti-DNA and anti-hapten antibodies exhibited the Igb allotype and thus were produced by the F1 donor B cells. The initial phase of tolerance induction was apparently associated with an allogeneic helper effect, because DNP-KLH-primed F1 donor cells transferred to newborn BALB/c could be stimulated after challenge with DNP-BGG. The triggering of persisting auto-reactive F1 donor B cells may reflect an activation by "incompletely" tolerant semiallogeneic T cells.