The hydrolysis of oestrone sulphate to oestrone, by oestrone sulphatase, is thought to be the major source of the high oestrogen concentrations which are present in breast tumours. In this study, the possibility that the location of the tumour within the breast or the size of the tumour might influence oestrone sulphatase activity has been examined. Normal and malignant breast tissues were obtained from 2 pre- and 9 post-menopausal women after mastectomy. Oestrone sulphatase activity was measured in microsomes prepared from these tissues and related to the location and size of the tumour. Of the 11 mastectomy specimens examined, 4 of the 11 (36%) quadrants bearing the tumour had the highest oestrone sulphatase activity. No significant difference in oestrone sulphatase activity was detected between normal and malignant tissues. No significant relationships were found between tumour sulphatase activity and activity in the quadrant bearing the tumour or tumour size and tumour oestrone sulphatase activity. The detection of the highest oestrone sulphatase activity in 36% of the breast quadrants bearing a tumour, or on which a tumour impinged, which a tumour impinges, while lending some support to the concept that breast tumours produce factors which are able to influence the activity of this enzyme, suggests that it is to a lesser extent than previously reported for the aromatase.