To determine whether aging and thymic involution are associated with defects in intrathymic T cell selection or clonal instabilities, we compared transcript levels for 18 V beta genes in thymic and splenic T cells of young (2 month), adult (12 month), and old (20 month) mice of various Mls and MHC haplotypes (C57BL/6, BALB/c, and DBA/2). The results showed that the unselected thymic V beta repertoires remain stable throughout life, despite severe reduction in cellularity in the thymus of aged mice. Similarly, splenic CD4 and CD8 V beta repertoires showed no significant alterations, and no leakage to the periphery of endogenous superantigen-reactive V beta clones was observed with age, even in irradiated and bone marrow-reconstituted old mice. Responses in vitro to bacterial superantigens were undiminished with age but, significantly, some of these superantigens expanded V beta clones that are cross-reactive with endogenous superantigens and are normally partially (V beta 11 and -12) or severely (V beta 3.1) deleted in BALB/c mice. In the course of these studies, several previously unrecognized reactivities of V beta with staphylococcal toxins were also revealed.