Under physiological conditions, the vast majority of T cells differentiate in the thymus, an organ that provides an optimal microenvironment for T cell maturation and shapes the T cell repertoire via positive and negative selection processes. In the present report, we demonstrate that neonatal thymectomy of CBA/H mice results in a diminution of T cells in peripheral lymphoid organs (spleen, lymph nodes), but is followed by a marked transient (12 wk) increase in Thy-1+ CD3+ cells in the peritoneal cavity. These cells exhibit predominantly a double-negative (CD4-CD8-) phenotype among which products of the T cell receptor (TCR) V beta 11 gene family (i.e., an I-E-reactive TCR normally deleted in I-E-bearing CBA/H mice) are selectively overexpressed. This observation suggests that, under athymic conditions, T cell differentiation and/or accumulation may occur in the peritoneal cavity. Intraperitoneal inoculation of an interleukin 2 (IL-2) vaccinia virus construct that releases high titers of human IL-2 in vivo induces conversion of these double-negative T cells to either CD4+ CD8- or CD4- CD8+ single positives, and allows in vitro stimulation of TCR V beta 11-bearing cells with a clonotypic anti-V beta antibody. Since IL-2 induces autoimmune manifestations (DNA autoantibodies, rheumatoid factors, and interstitial nephritis) in thymectomized CBA/H mice, but not in sham-treated littermates, this lymphokine is likely to enhance the autoaggressive function of T cells that bear forbidden, potentially autoreactive TCR gene products and that are normally deleted in the thymus.