Urethan leukemogenesis was found to be fairly potent in C57BL mice when the treatment was begun soon after birth (35% incidence) but very weak when the treatment was begun at about 45 days of age (6% incidence). The inhibitory effect of thymectomy on urethan leukemogenesis was not reversed by subsequent implantation of syngeneic newborn thymus, in contrast to such a reversal in the case of radiation leukemogenesis. Possible reasons for the difference are discussed.


This investigation was supported by Research Grant CA-05455 of the National Cancer Institute, USPHS.

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