AbstractWe have analyzed the origin and development of unusual CD4−CD8− α/β T cell receptor‐positive peripheral T cells produced in large numbers by mice homozygous for the gld mutation (C3H‐gld/gld). These mice may be an important model for investigating processes controlling T cell development. Bone marrow transfers demonstrated that the gld defect was intrinsic to bone marrow‐derived cells. Clonal deletion of potentially autoreactive cells was observed in peripheral gld CD4−CD8−, CD4+CD8−, and CD4−CD8+ T cells, as well as mature thymocytes. This suggests that gld CD4−CD8− T cells have passed through the thymus in ontogeny and that gld autoimmunity does not result from a general defect in elimination of self‐reactive thymocytes. These observations, combined with demethylation of the CD8 gene in the CD4−CD8− population, support prior expression of CD4 and/or CD8 in gld CD4−CD8− T cell ontogeny, perhaps at a CD4+CD8+ stage. Steroid sensitivity of gld thymocytes and CD4−CD8− T cells was normal. Therefore, we found no gross abnormalities in two major mechanisms of inducible cell death in the gld thymus, the clonal deletion process associated with tolerance and the steroid‐inducible endogenous endonuclease thought to be involved in apoptosis of unselected thymocytes. The data suggest that if gld CD4−CD8− T cells arise via escape from normal elimination in the thymus, they must do so by a novel defect in thymic selection (perhaps related to aberrant positive signals) and/or are expanded by an extrathymic process which allows clonal deletion to occur.