MATERIALS AND METHODS

SUMMARy-Strain BL/LyDe mice have been shown to possess the mammary tumor virus. Its presence has been demonstrated by the occurrence of mammary tumors in agent-free C3HfB and C3HeB females foster-nursed by strain BL. The virus has not been demonstrated in strain SWR/LyDe by the same procedure.-J Nat Cancer Inst 45: 215-218,1970. IN ATTEMPTS to demonstrate the presence of the mammary tumor virus (MTV) in strains BL/LyDe and SWRfLyDe mice (hereafter referred to as BL and SWR), the occurrence of mammary tumors was studied in females of agent-free substrains C3Hf/HeDe and C3HeB/De, foster-nursed by strains BL and SWR