Long-lived mammals such as humans respond over decades with CTL and helper T lymphocytes to acute and chronic infections. Maintaining these extended recall responses requires established memory populations of sufficient size and diversity to effectively respond to these infections. Studies in mice have indicated that cytotoxic T cells that respond to secondary viral infections are principally those that were activated in primary responses and maintained through memory. However, long-term recall responses in humans must involve many more responses with increased opportunities for recruitment of naive T cells and competition between memory and naive cells. We hypothesized that increased numbers of antigenic challenges prolong selection pressure on responding T cells resulting in continuously changing populations characterized by evolving TCR repertoires. This hypothesis was tested by transplanting recipients with 10 sets of H4-incompatible skin allografts that were harvested at times of rejection for spectratyping of TCR alpha and beta transcripts expressed by graft-infiltrating T cells. Amplicons with single complementarity-determining region 3 (CDR3) lengths were sequenced, and CDR3-specific primers were used for amplifications to identify graft sets in which these sequences were present. The results showed that TCR repertoires did not stabilize in these extended recall responses since the majority of identified alpha and beta CDR3s was present in varying numbers of graft sets with only a minority being present throughout. Selection for specific beta CDR3s was suggested by observed associations between the extent of the recall responses and shortened CDR3 lengths and restricted distributions of net charges in CDR3s.