To define age-associated alterations in the immune system at the molecular level, we have analyzed TCR Vβgene expression patterns at the fetal, neonatal, adult, and advanced ages of mice. In contrast to Vγand VHgenes, Vβgenes rearranged without any preference related to their chromosomal organization. Endogenous superantigen-mediated clonal deletions were registered for the first time at the neonatal stage, presumably reflecting the late developmental appearance of these molecules. Such deletions, once established, were maintained throughout life with little, if any, leakage in this process. Furthermore, bone marrow transplantation and other studies indicated that an involuted thymus maintained its capacity to perform both its functions, i.e. positive and negative selection. Although overall Vβrepertoires showed remarkable stability with advanced age, modifications in expression levels for some Vβ, particularly those associated with the CD8 subset and presumably reflecting antigenic stimulation, were recorded. Mice with lupus and early-life thymic involution were fully capable of deleting endogenous superantigen-reactive Vβclones, and even lupus mice with a genetic defect in the apoptosis-promotingFasgene were normal in this regard. The results indicate that, aside from some anticipated clonal expansions induced by antigenic stimulation, age-associated alterations in immune functions are not caused by any profound changes in the overall TCR repertoire.