The effect of T cells on T‐independent polyclonal B cell activation (PBA) was studied in vitro in mice. Helper/inducer T cells were activated by interaction with la antigens expressed on B cells. The activated T cells enhanced IgG synthesis in the PBA reaction. la antigen expression on B cells increased when stimulated by polyclonal B cell activators. An autoreactive T cell line was established by longterm culture of spleen cells with polyclonal B cell activators. The culture supernatant from this autoreactive T cell line enhanced IgG synthesis in PBA. The above results suggest that autoreactive T cells were activated by la expressed on B cells stimulated with polyclonal B cell activators, and then prompted B cells to synthesize IgG. Therefore, autoreactive T cells may play an important role in the establishment of the IgG plasma cell‐rich periodontal les