Dexter has described a culture system in which pluripotent stem cells (CFUs) are maintained for several months on an adherent layer derived from bone marrow. The population of cells floating in the supernatant medium contains stem cells and other cells at later stages of myeloid differentiation. Mature lymphocytes have not been observed in these cultures. Since the culture conditions may select for stem cells restricted to myeloid differentiation, we have studied the differentiative potential of cultured stem cells. Cultures were established using various combinatins of CBA bone marrow and CBA/HT6T6 bone marrow to distinguish between adherent layer derived cells and "recharge" derived cells. After various intervals in culture, cells were injected into irradiated recipients of either CBA or CBA/HT6T6 genotype. Three to four months after reconstitution, mitotic spreads were prepared from proliferating lymphoid and myeloid cells. The results showed that cells capable of giving rist to both B and T lymphoid progeny were maintained in culture for six weeks. The origin of the nonadherent CFUs in these cultures were also examined. They were derived from both the adherent and the "recharge" cell populations.