[Probable nature of a cell population facilitating splenic colony formation]

It has been disclosed that only syngeneic thymocytes are capable of stimulating colony-formation by the bone marrow treated with rabbit anti-mouse brain serum (RAMBS). The treatment of syngenic thymocytes with anti-theta-serum produces a negligible lowering of this effect. Meanwhile the treatment of the donor thymus with Con A 48 hours before the collection of thymocytes completely abolished the ability of these cells to stimulate colony-formation. The evidence suggests that the bone marrow may include a cell population with surface antigens reacting with RAMBS but not with anti-theta-serum which functions as a trigger in relation to CFUs. These cells seem likely to be precursors of T lymphocytes.