TPDMT-4 mouse mammary tumors have been characterized by pregnancy-dependent growth in breeders; formation of ductal, lobular, and alveolar structures in gland-free but insignificant outgrowth in intact fat pads of virgins; and continuous growth in hosts implanted s.c. with estrogen-progesterone pellets. Tumors were passaged in breeding and estrogen-progesterone-treated mice for 30 and 28 generations, respectively. The latter subline was called TPDMT-4EP. Both lines were compared for tumor-producing capability in virgins. Transplants from growing TPDMT-4 and growing and regressing TPDMT-4EP tumors were implanted into cleared and intact fat pads of 3-week-old mice and investigated for tumor formation for 12, 4, and 4 months, respectively. No significant differences were noted between cleared and intact fat pads. TPDMT-4 transplants formed significant tumors in 26 (72%) of 36 fat pads after a mean latency period of of 204 days. Growing and regressing TPDMT-4EP transplants formed significant tumors in 47 (98%) of 48 and in 27 (90%) of 30 fat pads, respectively, after a mean latency period of 52 days. Of eight TPDMT-4 tumors tested for ovarian dependence, six were ovarian dependent, one was ovarian responsive, and one was undecided. In contrast, of 31 TPDMT-4EP tumors tested, three were ovarian responsive, and 28 were ovarian independent. Five of seven independent tumors from ovariectomized mice still responded to the estrogen-progesterone pellet. Cytoplasmic estrogen and progesterone receptors were assayed in tumors different in ovarian dependence without noting significant correlation between their levels and ovarian dependence or growth rate. The results suggest that continuous hormone stimulation may enhance progression from dependence to autonomy of hormone-dependent tumors.