The CD19 cell surface molecule regulates signal transduction events critical for B lymphocyte development and humoral immunity. Increasing the density of CD19 expression renders B lymphocytes hyper-responsive to transmembrane signals, and transgenic mice that overexpress CD19 have increased levels of autoantibodies. The role of CD19 in tolerance regulation and autoantibody generation was therefore examined by crossing mice that overexpress a human CD19 transgene with transgenic mice expressing a model autoantigen (soluble hen egg lysozyme, sHEL) and high-affinity HEL-specific IgM^a and IgD^a (Ig^HEL) antigen receptors. In this model of peripheral tolerance, B cells in sHEL/Ig^HEL double-transgenic mice are functionally anergic and do not produce autoantibodies. However, it was found that overexpression of CD19 in sHEL/Ig^HEL double-transgenic mice resulted in a breakdown of peripheral tolerance and the production of anti-HEL antibodies at levels similar to those observed in Ig^HEL mice lacking the sHEL autoantigen. Therefore, altered signaling thresholds due to CD19 overexpression resulted in the breakdown of peripheral tolerance. Thus, CD19 overexpression shifts the balance between tolerance and immunity to autoimmunity by augmenting antigen receptor signaling.