Using reverse transcriptase-polymerase chain reaction amplification it was possible to detect the presence of type 1 human estrogen sulfotransferase (hEST1) mRNA in the hormone-dependent: MCF-7 and T-47D, and hormone-independent: MDA-MB-231 and MDA-MB-468, human breast cancer cells. The expression of this mRNA is significantly higher in the MDA-MB-468 cells and a correlation of this mRNA expression with the enzymatic activity was observed. The progestin promegestone (R-5020) at a low concentration (5 x 10(-7) M) can significantly increase the estrogen sulfotransferase activity and its mRNA in the hormone-dependent MCF-7 and T-47D cells. As estrogen sulfates are biologically inactive, the stimulatory effect on sulfotransferase by promegestone may open attractive possibilities in the control of estradiol in human breast cancer.