TPDMT-4 tumors showed significant growth in pregnant mice but not in virgins. Tumor pieces at transplant generation 14 were implanted into the gland-free (right) and the intact (left) fat pads in virgin mice and followed over a one-year period. The transplants formed outgrowths similar to normal mammary glands in the cleared fat pads. The outgrowths were hyperactive when compared with host glands and varied from one mouse to another in extent of development. In contrast, they did not outgrow and remained as traces among host glands in the intact fat pads. Tumors developed 41 and 32 weeks after implantation in the right and the left fat pads, respectively, in one of 18 mice. The tumor from the cleared fat pad was an ovarian-sensitive pale cell carcinoma, and contained estrogen receptors (ER) but not progesterone receptors (PR). That from the intact fat pad was a type B carcinoma which was ovarian-dependent and contained both ER and PR at levels comparable to those of the parent tumor.