We describe a system for growing murine colonies containing subsets of functional T cells derived from immature T cell precursors. In contrast with previously described systems in which colony formation appears to depend upon activation and clonal expansion of mature T cells, we obtain comparable or increased numbers of colonies after depletion of T cells from spleen or bone marrow, or on culturing spleen or bone marrow from athymic nude mice. The proportion of cells expressing the T cell

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... e markers Thy-1, Lyt-1, and Lyt-2 is highly variable from colony to colony, consistent with the colonies arising from immature T cells, which differentiate in the colonies to give rise to various subsets of mature T cells.