RAP mice, possibly harboring mammary tumor virus (MTV), of representative ages were exposed to limited or lifetime whole-body continuous gamma and to fractionated x-irradiation. Doses varied from 30-2100 R and dose rates from 0.062-121 R/day in the two series GA and PN in which mammary cancer incidence was elevated. Induction of mammary cancer was found to be related to the dose range for ovarian tumorigenesis, 30-500 R, but unrelated to age when exposed. There was no statistically significant correlation between mammary cancer and ovarian tumors. Active MTV infection was suggested by cancer-prone litters in one irradiated (PN U + SR) and one unirradiated series (OCB). Unirradiated descendents of perinatally irradiated mice with an inherited hormonal disorder had a lower incidence of mammary cancer than stock virgin mice. The balance of evidence pointed to induction of mammary cancer through systemic change set in motion by irradiation.

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