Cell to cell interaction, which plays a crucial role in breast cancer growth, may be regulated by steroid hormones. This study examined dihydrotestosterone (DHT) effects on the interaction between the steroid receptor positive MCF-7 and the steroid receptor negative MDA-MB-231 breast cancer cell lines. The growth of MDA-MB-231 cells was inhibited by medium conditioned by MCF-7 cells grown in presence of DHT but not by medium conditioned by MCF-7 cells grown in presence of both DHT and the antiandrogen hydroxyflutamide. Trypsin pretreatment of conditioned medium abolished its growth-inhibitory effect on hormone-unresponsive cells. DHT itself did not affect the growth of MDA-MB-231 cells when directly added to their culture medium. Data suggest that DHT stimulates, via the androgen receptor, the androgen-responsive breast cancer cells to produce a peptide factor(s) capable of inhibiting the growth of hormone-unresponsive cells.