Tumor cells of the ZR-75-1 line (1 X 10(7) in 0.5 ml) were inoculated into the right thigh muscles of BALB/c female nude mice which were, at the same time given 1.0 mg of estradiol subcutaneously. After the transferable strain had been established, the tumors were then transplanted into female and male nude mice either with or without estradiol treatment. Although no exponential tumor growth was observed in the untreated male and female nude mice, no complete rejection was found during the experiments. The estrogen receptor of this strain was positive and the growth of ZR-75-1 was dependent on exogenous estradiol. ZR-75-1 in the nude mouse was insensitive to tamoxifen treatment, given as 5 mg/kg intramuscularly twice a week, suggesting that dependency on estradiol is not necessarily correlated with endocrine sensitivity to tamoxifen.