The developmental potentialities of early mouse fetal thymocytes were analyzed by in vitro culturing cell suspensions obtained from 12-15-gestational day thymus in contact with a bone marrow stroma clonal cell line that supports pre-B and myeloid cell differentiation. B cell and macrophage development from fetal thymocytes was observed at all fetal stages tested, but limiting dilution analysis revealed that the frequency of cells forming colonies on bone marrow stroma is the highest in the fetal thymus at day 12, then dramatically decreases until day 15. These observations suggest that the thymus rudiment is seeded by multipotential precursor cells which are not immediately committed to T cell development in the thymic cellular environment.