The metabolism of various unconjugated oestrogens [oestrone (E1), oestradiol (E2) and oestriol (E3)], in MDA-MB-468 human mammary cancer cells, a cell line characterised by the absence of oestrogen receptor has been investigated. 24 h after incubation of these cells with 5 x 10(-9) mol/l of tritiated E1, E2, or E3, 62-90% of the total radioactivity was localised in the sulphate fraction. Analysis showed that 73-90% of the sulphate fraction corresponds with the oestrogen incubated. The formation of the oestrogen sulphates is rapid and maximum values are found after 3 h incubation. Intense sulphotransferase activity was also found for testosterone, pregnenolone and dehydroepiandrosterone. Thus these cells contained high oestrogen sulphotransferase activity, suggesting that the presence of high levels of oestrogen sulphates in breast cancer can be synthesised in the tumour itself. In addition, the control of this enzymatic activity could open new possibilities in the knowledge of oestrogen responses and of therapeutic applications in breast cancer.