The tumor cells (0.5 ml, 1×10⁷) of MCF-7 line were inoculated into the subcutaneous tissue or intraperitoneum of female BALB/c nude mice. Primarily tansplanted mice were treated with 17β-estradiol dipropionate (E₂) in a dose of 5 mg/kg and 17α-hydroxy progesterone caproate (Pg) in a dose of 250 mg/kg once a week. After the transferable strain was established, tumors were transplanted into female and male mice treated with E₂, Pg, and E₂+Pg. The tumors treated with E₂ or E₂+Pg grew exponentially while tumors in the other group regressed. Pg was assumed to play some role in the growth of MCF-7, in the presence of estrogen. Although cytosol estrogen receptors (ERc), nuclear estrogen receptors (ERn), and progesterone receptors (PgR) were detected by dextran coatedcharcoal method and exchange assay in the growing tumors, ERn and PgR of regressing tumors was usually negative. This MCF-7 strain in nude mice may be a promising animal model for studying chemo-hormone therapy for human breast carcinomas.