Mice homozygous for the autosomal recessive gene lpr develop marked lymphadenopathy and a systemic autoimmune disease resembling human systemic lupus erythematosus. The enlarged nodes are dominated by T cells with an unusual surface phenotype: dull Thy-1+, dull CD3+, CD4-, CD8-, B220+ (double-negative T cells or DNTs). Despite their massive accumulation in vivo, these cells fail to proliferate in response to conventional T-cell mitogens in vitro. The identification of the lpr mutation as a defect in the Fas apoptosis receptor gene suggests that DNT accumulation may result from abnormal persistence rather than overproliferation. To test in vivo whether DNTs persist abnormally or have a capacity to differentiate into single-positive T cells, we have performed cell transfer experiments between congenic strains of lpr and +/+ mice differentially marked by expression of the Ly-1 or Thy-1 alleles. Although transferred lpr lymph node cells were mostly DNTs at the time of injection, most recovered cells of donor origin were single positive, particularly CD8+, at all time points after transfer. Furthermore, transfer of purified DNTs resulted in recovery of relatively few cells of donor origin. Transfer of lpr T cells enriched for CD8 expression confirmed the preferential survival of this subset. Thus, DNTs are a surprisingly transient population and have little capacity for transformation to single positives. This would suggest that DNTs are constantly being renewed, perhaps from CD4+ and CD8+ precursors.