Using reverse transcriptase polymerase chain reaction amplification it was possible to detect the presence of oestrogen sulphatase mRNA in different hormone-dependent (MCF-7, T-47D) and hormone-independent (MDA-MB-231, MDA-MB-468) mammary cancer cell lines. The expression of this mRNA is significantly higher in T-47D and MDA-MB-231 than in the other cell lines, and a correlation of this expression with the enzymatic activities was observed. The progestagen Promegestone (R-5020) can significantly decrease the mRNA of the sulphatase in MCF-7 cells. As this progestagen can also inhibit the enzyme itself in the same mammary cancer cell line, it is suggested that for the decrease in the sulphatase activity not only the effect on the enzyme, but also the effect on transcriptional factor(s) which express this enzyme are involved. The present data not only contribute to the knowledge of the mechanism of the sulphatase activity, but also can open new possibilities in breast cancer treatment.