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THE origin of this strain of mice has been described by Foulds (1947, 1949a) who mated C57 Black females with males from a strain (RIII) carrying the mammary tumour virus. The offspring from some of these crosses developed pregnancy-dependent mammary tumours, and 1 such female became the founder of the present BR6 colony, which has been maintained by brother-sister mating for 74 generations. A characteristic of these tumours is that initially they appear and grow only during pregnancy, although eventually they become autonomous and independent of pregnancy. Tumours may also appear in virgin mice, usually when they are more than 1 year old. Mundy and Williams (1961) reported the occurrence of several tumour-free families in the BR6 colony. From 1 of these was established the non-tumourbearing (NT) subline which has been maintained separately from the tumourbearing (T) subline for 45 generations. The existence of this subline provided an opportunity to investigate the relative importance of the virus and its environment in tumour production and behaviour. This has been done in 3 ways. 1. Mice were deprived of the mammary tumour virus, or exposed to it, or exposed to one different from that carried by their own mothers, by cross-suckling on foster-mothers of a different subline or strain. 2. The genetic constitution of the mice was altered by hybridization between the sublines. 3. Mice obtained from the first 2 groups of experiments were used to study the influences of the mammary tumour virus and genetic background on the growth of transplanted pregnancy-independent tumours.

Genetic and viral influences of mammary tumours in BR6 mice