In addition to thymic T cell selection, post-thymic mechanisms of tolerance induction are required to eliminate autoreactive T cells with specificities for peripheral self antigens. While CD8+ T cells can recognize their target antigen on a wide variety of cell types, CD4+ T cells generally depend on the presence of specialized antigen-presenting cells. Because of this fundamental difference in antigen recognition peripheral tolerance of CD4+ T cells appears more difficult to achieve than of CD8+ T cells. Utilizing T cell receptor (TCR)-transgenic mice in which CD4+ T cells specific for a pancreatic β cell neoantigen (the simian virus 40 T antigen) are constantly generated at low frequency, we have now established a mouse model of peripheral, tissue-specific CD4+ T cell tolerance. In these animals, tolerance is preceded by a phase of activation of the autoreactive T cells as characterized by up-regulation of CD69 and CD44, and down-regulation of the L-selectin lymph node homing receptor. T antigen-specific T cells bearing this phenotype can be detected in the local lymphoid environment of the pancreas but not in more remote locations like axillary or inguinal lymph nodes. The proportion of activated, autoreactive T cells is maximal at 2–3 weeks of age, after which these cells are gradually deleted from the peripheral lymphocyte pool. We further demonstrate that deletion of the autoreactive T cells does not occur in TCR-tansgenic mice bred to the RAG-1-deficient background in which the transgenic T cells represent the only functional lymphocyte population.