We identified committed T cell progenitors (CTPs) in the mouse bone marrow that have not rearranged the TCRbeta gene; express a variety of genes associated with commitment to the T cell lineage, including GATA-3, T cell-specific factor-1, Cbeta, and Id2; and show a surface marker pattern (CD44+ CD25- CD24+ CD5-) that is similar to the earliest T cell progenitors in the thymus. More mature committed intermediate progenitors in the marrow have rearranged the TCR gene loci, express Valpha and Vbeta genes as well as CD3epsilon, but do not express surface TCR or CD3 receptors. CTPs, but not progenitors from the thymus, reconstituted the alphabeta T cells in the lymphoid tissues of athymic nu/nu mice. These reconstituted T cells vigorously secreted IFN-gamma after stimulation in vitro, and protected the mice against lethal infection with murine CMV. In conclusion, CTPs in wild-type bone marrow can generate functional T cells via an extrathymic pathway in athymic nu/nu mice.