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BR 6 mice carry a milk-transmitted mammary tumour virus, and breeding females of this strain spontaneously develop mammary tumours, which are at first pregnancy-dependent. That is, they grow during pregnancy, regress completely or partially after parturition and reappear, or start to grow again, during the next pregnancy (Foulds, 1947, 1949). This suggests that tumour growth is influenced by the hormones of pregnancy. After a variable number of pregnancies, regression becomes less until eventually the tumours apparently grow independently of further stimulation.

A tumour-free family, which had lost the mammary tumour virus, occurred within the strain (Mundy & Williams, 1961) and has been maintained as a separate subline for 45 generations.

To investigate whether pregnancy-independent tumours were completely hormone-independent or whether some hormone stimulation was still required, they were transplanted into two groups of castrated male mice of the tumour-free subline. Males of one group were given oestrone (100 ng/ml) in