Estrogen is a negative regulator of lymphopoiesis and provides an experimental tool for probing relationships between lymphocyte precursors and stem cells. We found that expression of lymphocyte-associated genes and immunoglobulin (Ig) gene rearrangement occurred before CD45R acquisition. Lymphoid-restricted progenitors that were Lin(-)IL-7R alpha(+)c-kit(lo)TdT(+) (lineage marker(-), interleukin receptor 7 alpha(+), c-kit(lo) and terminal deoxynucleotidyl transferase(+)) were selectively depleted in estrogen-treated mice; within a less differentiated Lin-c-kit(hi) fraction, functional precursors of B and T, but not myeloid, cells were also selectively depleted. TdT and an Ig heavy chain transgene were detected within a hormone-regulated Lin(-)c-kit(hi)Sca-1(+)CD27(+)Flk-2(+)IL-7R alpha(-) subset of this multipotential progenitor population. Identification of these extremely early lymphoid precursors should facilitate investigation of the molecular mechanisms that control lineage-fate decisions in hematopoiesis.