Tumors could escape an immune attack by inducing peripheral T cell tolerance. To test this, T cell receptor (TCR)‐transgenic mice were injected with plasmacytoma cells secreting a highly tumor‐specific antigen, a monoclonal immunglobulin (Ig), for which the transgene‐encoded TCR is specific. The TCR recognizes a third hypervariable region idiotypic (Id) peptide of the Ig, presented by a class II molecule on host antigen‐presenting cells. The TCR‐transgenic mice have previously been shown to be protected against an Id⁺ plasmacytoma challenge. In the present experiments, the protection was deliberately overwhelmed by subcutaneous injection of large numbers of plasmacytoma cells. Such tumor mice, chronically exposed to increasing amounts of monoclonal Ig, delete Id‐specific CD4⁺ T cells in their peripheral lymphoid organs and in the tumor. The residual CD4⁺ cells express endogenous, rather than transgene‐encoded TCR α chains. Peripheral deletion, functional T cell unresponsiveness, and thymocyte deletion are all first detected at the same serum concentration of monoclonal Ig, ∼50 μg/ml (0.3 μM), and become more and more profound as the tumor burden increases. The results suggest that peripheral T cell tolerance to Id could be a tumor escape mechanism in patients with B cell malignancies. In addition, the findings have implications for T cell tolerance to Ig V regions in normal individuals.