Treatment of murine marrow with triethylenethiophosphoramide, triethylenemelamine, Myleran, or urethan severely depresses the ability of bone marrow to form spleen colonies in heavily irradiated animals. No age difference was observed with respect to the effects of urethan pretreatment of 30- and 90-day-old animals in this regard. Myleran and thio-TEPA, while inhibiting spleen colony-forming ability, were without effect on erythropoiesis in donor animals. These results have been compared with results obtained from other studies in which these drugs, particularly urethan, were used to determine marrow's ability to promote thymic regeneration, prevent tumor formation, and prevent lethal irradiation death. The different results obtained in these experiments lead to the conclusion that the spleen colony-forming cell of marrow does not perform these functions.

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