THE discovery of the milk factor or mammary tumour agent (Bittner, 1936) and the demonstration that three factors, namely, the genetic factor, the hormonal factor, and the milk factor, are essential for the development of spontaneous breast cancer in certain inbred strains of mice (Bittner, 1939), was followed by investigations of the part played by the agent in the induction of breast cancer by oestrogenic hormone. Lacassagne (1939a), Lacassagne and Danysz (1939), Twombly (1939Twombly ( , 1940, and later Bittner (1940Bittner ( , 1941, Gardner (1941a), Shimnkin andAndervont (1941, 1942), Dmochowski and Gye (19.44), found that fosternursing of low-breast-cancer strain mice by high-breast-cancer strain females increases the incidence of breast cancer in the low-breast-cancer strain mice treated with oestrogenic hormones. Tlhus male mice do not develop breast cancer, even if they are susceptible and have the mammary tumour agent, unless the hormonal or oestrogenic factor is supplied, and only few or no breast tumours are induced in low-breast-cancer strain mice, i.e. in the absence of milk factor, following treatment with oestrogenic hormones. It is probable that a quantitative deficiency of any one factor can be overcome by a relative quantitative increase in the other two factors (Shimkin, 1945).The accelerating influence of carcinogenic hydrocarbons on the appearance of breast cancer in mice of unmknow-n genetic constitution was first observed by Maisin and Coolen (1936) and Perry and Ginzton (1937), while DobrovolskaiaZavadskala and Adamova (1938, 1939), were unable to observe a similar effect on inbred mice employed by them. Experiments carried out by Mider and Morton (1939), Bonser and Orr (1939), Strong and Smith (1939), Bonser (1940), Strong and Williams (1941), Engelbreth-Holm (1941), Engelbreth-Holm and Lefevre (1941), Orr (1943), Kirschbaum, Lawrason, Kaplan and Bittner (1944), Kirschbaum, Williams and Bittner (1946), Shimkin and Bryan ), Strong (1945), Orr (1946, demonstrated that carcinogenic hydrocarbons induce the appearance of breast cancer in females of low-cancer strains or accelerate the development of breast tumours in female mice of high-cancer strains. In these experiments it was also shown that oestrogenic influence is important in the induction of breast cancer by carcinogenic hydrocarbons, since mice developing these tumours in the absence of treatment with oestrogens were females, with one exception, a CBA male (Orr, 1943). The strains employed, the various routes by which the carcinogenic hydrocarbons were given and the results obtained by different workers are presented in Table I.