The recent description of an early T-lineage progenitor (ETP) population in adult mouse thymus implies the presence of a bone marrow predecessor that has not yet been identified. Here we describe a Lin^Neg Sca-1^Pos c-kit^Hi Thy-1.1^Neg L-selectin^Pos adult mouse bone marrow population that resembles the thymic ETP in both antigen expression phenotype and posttransplantation lineage potential. These cells produce wavelike kinetics of thymic seeding and reconstitute the irradiated thymus with kinetics comparable to a thymocyte graft after intravenous transplantation. Transient B-lineage reconstitution is also observed, but little myeloid potential can be detected in transplant experiments. A second subset of progenitors is L-selectin^Neg and is highly enriched for rapid and persistent T- and B-lineage potential, as well as some myeloid potential. L-selectin (CD62L) is therefore an effective marker for separating lymphoid progenitors from myeloid progenitors and hematopoietic stem cells in mouse bone marrow. (Blood. 2004;103: 2990-2996)