The 13762 mammary adenocarcinoma was subjected to serial transplantation in rats, freezing and thawing and in vitro cultivation. Following these procedures, various sublines were identified which differed in histological pattern, oncogenicity and sensitivity to medroxyprogesterone acetate treatment. Medroxyprogesterone acetate, administered s.c. at the dose of 100 mg/kg/day, 5 days/week for 4 weeks, delayed tumor onset and growth of sublines that had maintained their sensitivity to 17-β-estradiol. Doxorubicin, administered i.v. at the dose of 3.6 mg/kg once a week for 4 weeks, was more active on the hormone-sensitive than on the hormone-insensitive subline. Simultaneous combined treatment for 4 weeks with these agents caused higher growth inhibition than either single agent given alone. Other schedules of treatment were less effective. The following conclusions can be drawn: (1) the 13762 mammary tumor is heterogeneous, and hormone-sensitive or insensitive cell populations can be selected by simple manipulations; (2) there is no correlation between histological pattern and hormone sensitivity; (3) medroxyprogesterone acetate is active on sublines sensitive to 17-/3-estradiol treatment, but it also improves the activity of doxorubicin on hormone insensitive sublines.