Currently there is much interest in the role that growth factors may play in the development of human breast tumours. We have shown previously that growth factors secreted by breast tumours may influence the activity of oestradiol hydroxysteroid dehydrogenase, the enzyme which catalyses the interconversion of oestrone (E1) and oestradiol. As the formation of E1 from its sulphate (E1S) by oestrone sulphatase may be quantitatively more important than production from androstenedione via aromatase, we have studied the effect of insulin-like growth factor-1 (IGF-I) and basic fibroblast growth factor (bFGF) on oestrone sulphatase activity in the hormone-dependent MCF-7 and the hormone-independent MDA-MB-231 breast cancer cell lines. In both these cell types, bFGF (1-200 ng/ml) and IGF-I (25-200 ng/ml) significantly stimulated oestrone sulphatase activity in a dose-dependent manner (by 8-60%) after 48 h. Additionally, cycloheximide significantly inhibited (by 90-120%) this stimulation of oestrone sulphatase activity by the two growth factors in both MCF-7 and MDA-MB-231 cells. Basal oestrone sulphatase activity was higher in the oestrogen receptor, ER-ve MDA-MB-231 cells than in the ER + ve MCF-7 breast cancer cells. We conclude that these growth factors, believed to be secreted by breast tumours, may induce enzymes of oestrogen synthesis and hence increase local production of oestrogens.