Mammary lesions in susceptible, but mammary tumor virus (MTV)-free, BALB/c mice (C−) and in their progeny were analyzed after attempts were made to infect these mice with GR MTV. Ovaries of C− and GR females were transplanted into the ovarian capsules of (C− × GR)F1 mice, and these hybrids were mated with I or C− males. The C− progeny born of ovary transplants were judged MTV-free because of the failure of forced-bred females to develop mammary lesions or of the males to transmit virus to their progeny. These results indicate that the germ cells of C− females, even after prolonged exposure to a GR MTV-containing environment, fail to become infected with this virus. Of ≥3-week-old C− females inoculated with GR blood cells and forced-bred, 80% developed nodules and pregnancy-independent tumors. None of these mice developed plaques. Unfiltered plasma from GR donors inoculated into similar mice induced mammary adenocarcinoma in 31% of the mice, none of which, however, developed either nodules or plaques. Filtered plasma from the GR donors inoculated into mice induced no mammary lesions. Of the C− females mated with GR males 2–6 times, 47% contained nodules. Nodules resulting from blood cell inoculation and male transmission (semen) were transplanted into mammary-gland-free fat pads of 3-week-old C− females, which were subsequently forced-bred. These hosts developed only nodules and pregnancy-independent tumors, but their female progeny developed all 3 types of lesions including plaques. The results indicate that both blood cells and semen from GR mice contain MTV information for the development of both nodules and plaques. The nature of the lesions induced seems to depend on the age at which the host receives GR MTV.