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DIFFERENTIATION OF MOUSE T‐LYMPHOCYTES 1. CHARACTERIZATION OF THYMOCYTE PRECURSORS IN BONE MARROW BY ONE G VELOCITY SEDIMENTATION

Bone marrow cells were transferred into 700 RX‐irradiated syngeneic mice, and reganeration of the thymus was demonstrated by determining the total amount of DNA in the thymus. The amount of DNA in the regenerated thymus 9 days after X‐irradiation paralleled the number of cells injected, the range being 0 to 106 cells. These results indicate that regeneration during this period reflects the proliferation of thymocyte precursors existing in bone marrow cells, therefore such can be used as an indicator to determine the number of thymocyte precursors.