Previous results from several laboratories have demonstrated the presence of functional T lymphocytes in congenitally athymic (nude) mice. The present study represents an analysis of the T cell receptor repertoire exhibited by such cells. Clones of H-2Kb-specific cytotoxic T lymphocytes (CTL) were generated under primary limiting dilution conditions by using spleen cells from nude mice. These clones were analyzed on a panel of Kb mutant target cells to assess the receptor specificity of each clone. Unlike thymic bearing mice the CTL repertoires of which are exceedingly diverse, it was found that in most cases the vast majority of clones from each individual exhibited the same reactivity pattern. The particular pattern varied from individual to individual. Clones from three animals that exhibited this phenomenon were additionally analyzed by using a monoclonal antibody that can detect the utilization of the gene products of the V beta 8 family. In one animal all clones were V beta 8 positive, whereas in the others, all clones were negative. We conclude that the T cell receptor repertoire in nude mice is extremely limited and represents in vivo expansion of a relatively small number of functional precursors.