Cytotoxic T lymphocyte precursors (CTLp) from B10.D 2, B10.BR, and (B10.D 2 × B10.BR)F 1 mice of different ages have been activated by irradiated “wild-type” H2K b antigens (from B10.A(3R) mice) under limiting dilution conditions such that cytotoxic cells in responder wells represent the progeny of a single CTLp. After expansion in the presence of IL 2 and irradiated C57B1/6Kha spleen cells the contents of each well were divided into equal aliquots and tested for lysis with a panel of selected H2K b mutant targets. As has been observed for the murine B-cell repertoire, there seems to be substantially more homogeneity in the neonatal allo-T-cell repertoire than in the adult mouse. Furthermore, while the adult F 1 repertoire is markedly distinct from that expressed by either parental T-lymphocyte pool, the neonatal repertoire apparently reflects a relatively accurate composite of each parental population, codominantly expressed. These data, combined with studies of adult bone marrow radiation chimeras, suggest that during development of the adult T-lymphocyte repertoire from the initially expressed restricted (germ-line?) recognition specificities, somatic diversification driven by environmental (MHC?) antigenic determinants occurs. In addition to this ontogenetic development, during senescence another “regulation” of the repertoire becomes apparent, and once more the heterogeneity of recognition specificities is diminished. Nevertheless, the homogeneity seen in aged mice does not represent a simple return to the expression of the limited number of allo-specificities encoded in the neonatal repertoire.