In order to improve the knowledge of prolactin receptors (PRL-R) in human breast tumors, we studied PRL-R modulation by lactogenic and steroid hormones in the PRL-R rich human breast cancer cell-line, T-47 D. The PRL-R were assayed on a preparation of cell total membranes. We demonstrated an abnormal homologous in vitro regulation of PRL-R. Concentrations of human growth hormone (hGH) greater than 500 ng/ml were required to cause a decrease in PRL-R, with a maximal down-regulation at 2000 ng/ml and for 48 hours. Human placental lactogen (hPL) induced a decrease in PRL-R at concentrations greater than 500 ng/ml but later than hGH; ovine prolactin (oPRL) had no effect on PRL-R. Moreover, we also demonstrated that progestins specifically modulated the expression of PRL-R in T-47D cells: Org 2058, a synthetic progestin induced a statistically significant increase in PRL-R after a twenty-four hour incubation period: this effect was already observed at 10(-9) M and was maximal for 10(-6) and 10(-5) M (186% +/- 3.5% (+/- SEM) for total PRL-R). At 10(-6) M, the stimulation occurred early at three hours and was maximal at twenty-four hours. Conversely estradiol (10(-9) to 10(-6) M), cortisol (10(-9) to 10(-6) M), dexamethasone (10(-9) to 10(-5) M) and RU 486 (10(-9) to 10(-5) M), a progestin and glucocorticoid antagonist, had no effect on PRL-R levels. The Org 2058 PRL-R stimulation was abolished in the presence of RU 486. The abnormal PRL-R down-regulation in the human breast cancer cell-line, T-47D, may contribute a growth advantage to these malignant cells over normal tissues. The progestin PRL-R dependence suggests that high levels of PRL-R may reflect a functional progesterone receptor (Pg-R) and a highly hormone-dependent-phenotype of the tumor. These results support a potential role of PRL in the etiology of breast tumors and may have important implications in the management of human breast cancer.