In this study, we analyze the recall response to influenza A matrix peptide M1 (58-66) restricted by HLA-A2 in one individual and find a strict CDR3 selection as well as a high degree of polyclonality. The TCR β-chain repertoire of memory T cells specific for this Ag system has been shown previously to be constrained by the use of the BV17 family and the I/sRS A/S amino acid motif in the CDR3 region. Our sequence analysis of BV17 TCR from a CTL line showed the repertoire to be highly polyclonal, as 95 distinct CDR3 sequences (clonotypes) were identified expressing this CDR3 motif. The clonotype frequencies showed a power law distribution with an extensive low-frequency tail. The clonotypes present in the high-frequency component of the distribution could be measured directly in the PBMC. This measurement showed that the relative frequencies of these clonotypes before stimulation were similar to their frequencies after culturing. Analysis of short-term cultures showed that the responding clonotypes have a similar ability to proliferate, which is independent of TCR β-chain CDR3 sequence or precursor frequency. These data indicate that the memory T cell repertoire is composed of a surprisingly diverse set of T cell clonotypes with a limited potential for expansion. We propose that the high-frequency component represents T cells that have existed the longest. In keeping with this hypothesis, these clonotypes were measured over a 2-year period, during which their precursor frequency did not change.