Oestrogen sulphatase may play an important role in providing intracellular oestrogens from E,S for the growth and maintenance of breast tumours. In this study, we characterized oestrogen sulphatase in the hormone-dependent (ER/PR+) MCFJ and in the hormone-independent (ER/PR-) MDA-MB-23 I breast-cancer cells and, furthermore, examined its modulation by MPA, 4-OH-A4, tamoxifen, danazol, ethinyloestradiol and DHAS in both these cell types. Our detailed study of oestrogen sulphatase activity as a function of incubation time, E,S concentration and numbers of MCF-7 and MDA-MB-231 cells showed that more E,S was hydrolysed by MDA-MB-23 I cells than by MCF-7 cells at all time points and all substrate concentrations. Additionally, although the K, values of E,S for oestrogen sulphatase in both MCFJ and MDA-MB-231 cells were similar, the V, , values, and therefore the activity, differed greatly. The effect of various steroidal and non-steroidal compounds also suggested differences in these 2 cell lines with respect to oestrogen sulphatase inhibition or stimulation. MPA significantly increased the hydrolysis of [3H]E,S in both cell lines, possibly through i t s effect on membrane fluidity. Tamoxifen increased E,S hydrolysis in MDA-MB-23 I cells but not in MCF-7 cells, whereas 4-OH-A' inhibited E,S in MCF-7 cells but not in MDA-MB-23 I cells. Danazol (an isoxazol derivative of I7aethinyltestosterone), I7a-ethinyloestradiol and DHAS all significantly inhibited oestrogen sulphatase activity in both cell lines. Furthermore, danazol had a growth-inhibitory effect on both MCF-7 and MDA-MB-23 I cells, although MCF-7 cells appeared to be more sensitive t o growth inhibition by danazol.