Cells capable of forming colonies in spleens of irradiated mice (CFU) are lost temporarily when bone marrow cells from rats or mice are maintained in culture. Rat marrow CFU go through a minimum at about 3 days after which there is a slow increase in the number of CFU in culture, reaching a maximum at 9 days. Mouse marrow CFU reach a minimum at 3 days and a maximum at 7 days. Some rat marrow CFU persist in culture for as long as 28 days.

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