Specimens of human breast tissues from the periphery of excised benign tumors, obtained from 17 pre-menopausal patients, were processed into slices and transplanted subcutaneously to 122 female Balb/c athymic nude mice. Subsequently, the host mice were treated with estrogens, progesterone, thyroxine, and human placental lactogen (HPL) alone or in combination. Growth of the ductal epithelium within the human breast transplants, as a function of the hormone treatment, was assessed by 3H-thymidine autoradiographic analysis, i.e., the number of 3H-thymidine radiolabeled ductal cells per unit area of ductal epithelium (labeling index [LI]). The administration of estrogen or thyroxine alone significantly increased the LI. Progesterone or HPL treatments alone did not substantially influence LI. HPL treatment, but not progesterone or thyroxine treatments significantly enhanced the stimulatory effect of estrogen on LI. The athymic nude mouse as host to human breast tissues in these in vivo/ex vivo studies, has been instrumental in producing information that effectively increases understanding of the hormonal factors influencing DNA synthesis in human breast ductal epithelium.