The protooncogene MYC has been implicated in both the proliferation and programmed cell death of lymphoid cells, and in the genesis of lymphoid tumors. Here, we report that overexpression of MYC , as found in many lymphomas, can break immune tolerance. Mice that would otherwise be tolerant to a transgenic autoantigen mounted an immune response to the antigen if MYC was vigorously expressed in the B cell lineage. The responsive B cells converted to an activated phenotype and produced copious amounts of autoantibody that engendered immune complex disease of the kidney. MYC was required to both establish and maintain the breach of tolerance. These effects may be due to the ability of MYC to serve as a surrogate for cytokines. We found that the gene could mimic the effects of cytokines on both B cell proliferation and survival and, indeed, was required for those effects. These findings demonstrate a critical role for MYC in the response of B cells to antigen and expand the potential contributions of MYC to the genesis of lymphomas.