We analyzed the T cell receptor (TCR) rearrangements of 100 TCR-alpha/beta CD4-CD8- (double negative [DN]) T cell clones from normal individuals. We found that in four out of six donors this subset contains expanded clones that often account for 0.5% and, in one individual, even 7% of all peripheral blood lymphocytes. By combining limiting dilution analysis and N region oligotyping of polymerase chain reaction amplified TCR cDNA, we could measure the clonal size and show that two of these expanded clones remain stable in size for up to 4 yr in peripheral blood. The expanded clones analyzed ex vivo are not cycling and CD45 RAhi ROlo, but express high levels of alpha 4/beta 1 integrins, suggesting that they may have reverted to resting cells after activation. One of these expanded DN clones proliferates in vitro in response to Escherichia coli presented by monocytes cultured in GM-CSF plus IL-4 and kills CD1a+ Molt-4 cells. In contrast to what was found in the alpha/beta DN subset, alpha/beta CD4+ T cell clones specific for a tetanus toxin epitope showed a very small clonal size (< 1 in 10(7)) and could not be reisolated after 2 yr. Taken together, these results indicate that large clonal size and persistence are distinctive features of alpha/beta DN cells specific for bacterial antigens. These cells may use antigen-presenting cells, restriction molecules, and selection routes different from those used by antigen-specific CD4+ T cells.