Recent studies have documented incomplete TCR V alpha-chain allelic exclusion and dual V alpha-bearing T cells. Herein, we show that V beta allelic exclusion is also incomplete, since a significant proportion of peripheral T cells express dual V beta in both TCR transgenic and normal mice. Studies in TCR transgenic mice indicated that although a small proportion of T cells escaped allelic exclusion in the thymus, dual V beta-expressing cells expanded dramatically in the periphery with age, and such expanded cells had an activated phenotype. Although not as pronounced, age-related increases in dual V beta-bearing cells were also observed in normal mice. These findings may have important implications for TCR selection and specificity, age-related repertoire changes, and autoimmune disease pathogenesis.