Early stages of mammary tumors (EMT) were induced with a combined treatment of progesterone (P) and estrone (E) in ovariectomized adult GRS/A (GR) mice, a strain of European origin and with a high incidence of mammary cancer. The mammary tumors were comparable to the pregnancy-dependent tumors of breeding females of this strain. The hormone treatment did not lead to EMT in a variety of other strains and only occasionally in the RIII and C3H strains. Treatment with P or E alone did not lead to EMT in GR mice, but treatment with the steroid compound 17α-ethynyl-19-nortestosterone (ANT) did mimic the combined effect of P and E. Since EMT could be induced by ANT in all ovariectomized adult GR mice within 3 weeks, this tumor-induction method was suitable for analysis of the gene responsible for palpable, pregnancy-dependent mammary tumors of this strain. Another argument for the usefulness of this test for genetic analysis was the fact that, though mouse mammary tumor virus (MuMTV) of the GR strain was introduced into BALB/c and MAS mice by foster-nursing, ANT treatment did not lead to EMT. First and second backcross analyses showed that one gene was responsible for EMT induction. There was a strong correlation between the presence of EMT and MuMTV antigens in the mammary glands and milk of several first backcross populations between GR and other strains such as C57BL, BALB/c, DBAf, and C3Hf. This suggested that the expression of MuMTV antigens was also controlled by the EMT gene. Two types of resistance phenomena were observed. Neither type could prevent EMT after hormone treatment; however, they could delay EMT development. One resistance factor for EMT induction was noticeable and dominant in reciprocal hybrids of the GR and DBAf strains, whereas another resistance factor was detected in the backcross population only [i.e., in the C57BL × (C57BL × GR) backcross] and not in hybrids; therefore, this factor was recessive. Until now, linkage experiments with 18 markers to locate the gene for EMT induction in the map of the mouse were unsuccessful.