Tumor incidence and hair follicle lethality in rat skin were determined after various single and split doses of monoenergetic electrons produced by a Van de Graaff accelerator. In the split-dose groups, an initial dose of 1000 rads was followed 24 hr later by graded doses from 1000 rads to 4000 rads. The cumulative number of tumors per rat increased steadily in all groups consistent with a linear dependence on time after a somewhat variable tumor-free period. Hair-follicle survival was determined at death of the animal and dose displacement of the response curves was equivalent to about 80% of the initial dose. The curve of tumor response at 70 wk versus dose was peaked, but the ascending limbs of the curves were displaced by at least 70% of the initial dose. It was concluded that oncogenic recovery was too rapid for cell repopulation to be significantly involved and that most, if not all the observed recovery was based on a relatively rapid process similar to Elkind recovery.

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