The tumor cells (0.5 ml, 1 X 10(7) of MCF-7 line were inoculated into the subcutaneous tissue or intraperitoneum of female BALB/c nude mice. Primarily transplanted mice were treated with 17 beta-estradiol dipropionate (E2) in a dose of 5 mg/kg and 17 alpha-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 coated-charcoal 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.