Mammary tumors were induced in 16 of 37 (43%) adult female C57L/J X A/HeJ mice treated with mammary-tumor virus (Bittner''s MTA) and "mammotropins" (administered by means of a mammo-tropic pituitary tumor graft). In contrast, no mammary tumors developed in 159 female C57L/J X A/HeJ mice when treated with either "mammotropin," virus, or diethylstilbestrol alone or a combination of diethylstilbestrol and virus, or diethylstilbestrol and mammotropins. The mammary-tumor incidence was higher (61%) in mice given diluted milk (0.1 ml, 120) than in those giving undiluted milk (26%). The hormone responsiveness of 4 mammary tumors induced by virus plus mammotropin was tested by transplantation. One of these tumors was highly responsive to mammotropins; 3 were fully autonomous. The responsive tumor retained its full hormone dependency in the frozen state at -70''C. The latency period of hormone-responsive mammary tumor was related to the level of mammotropins. These findings support the concept that mammotropins are powerful promoters of mammary tumors but are not primary carcinogens. Induction of mammary tumors and their progressive growth are influenced by mammotropin levels, irrespective of whether the initiation is a chemical or ionizing radiation, as previously shown, or a virus, as shown in the present study.