We have previously produced a transgenic mouse line for hen egg lysozyme (HEL), an experimental model for analyzing tolerance to self-antigens at the peptide level. We have now characterized transgenic mice with HEL blood levels below 2 ng/ml, where significant T cell proliferative responses to HEL and its immunodominant peptide were observed. This HEL-low transgenic model was chosen because it mimics physiological conditions in which autoreactive T lymphocytes, recognizing self-components expressed at very low levels, persist without inducing a break in tolerance. Furthermore, in H-2d mice, HEL-specific T lymphocytes are triggered by a single immunodominant region, allowing us to compare the HEL-specific T cell V beta repertoires of transgenic and nontransgenic animals against a single peptide presented as self or foreign, respectively. We found that a V beta 8.2-D beta 1-J beta 1.5 rearrangement is found in response to HEL in all nontransgenic mice, whereas this V beta-restricted response is absent in HEL-low transgenic animals. At the nucleotide level, this rearrangement results from the trimming of the genomic segments during VDJ or DJ joining, without N additions, suggesting that the dominant rearrangement is selected early during fetal or neonatal life, before the expression of terminal deoxynucleotidyl transferase. In HEL-low transgenic mice, no dominant rearrangements are found as alternatives to the one observed in normal mice. Instead, each transgenic animal uses a different set of V beta-J beta combinations in its response to the immunodominant HEL peptide. In nontransgenic mice, besides the dominant V beta 8.2-D beta 1-J beta 1.5 combination, minor V beta repertoires were found which differed in each animal and were distinct from the rearrangements used by individual transgenic mice. These findings suggest that the T cell response to an immunodominant peptide involves a "public" V beta repertoire found in all animals and a "private" one which is specific to each individual.