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The incidence of radiation-induced myeloid leukemia, to which conventionally reared RF mice are unusually susceptible, has been observed to vary among colonies of RFM mice maintained in different microbial environments. The incidence in germ-free RFM males exposed to 300 R of X-irradiation at 5 to 6 weeks of age was only I %, whereas that in their conventionally reared counterparts was I3 %. In similarly irradiated RFM males subjected to microbial shock by transfer from an environment containing a limited number of bacteria to a conventional microbial environment, the incidence was 20%. The results suggest that the incidence of myeloid leukemia varies inversely in relation to rhe bacterial '' cleanliness " of the environment, possibly as a result of associated variations in granulocytopoiesis.

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