Recent experimentation has suggested that a decline in the proportion of lymphocytes that can respond to antigenic stimulation may contribute to the loss in immune function with increasing age. If a diminution of precursor frequency is a fundamental element of immunosenescence, then one would expect that manipulations which extend lifespan would also lead to a retarded decline in the measured proportion of reactive cell. Food restriction to about two-thirds of normal intake, initiated at weaning, has long been known to extend the lifespan of mice and rats. We show here that food restriction from weaning causes old mice to retain high levels of T cell precursors in both the helper and cytotoxic lineages. This association of delayed precursor cell loss with extended lifespan supports the hypothesis that changes in T cell precursor frequency may be a key component of age-related immunodeficiency.