SUMMARY: Tamoxifen (ICI 46,474) has been shown to possess anti-tumour properties in the dimethylbenz(a)anthracene (DMBA)-induced rat mammary carcinoma model. During tamoxifen therapy the binding of [3H]oestradiol in vivo to uterine (P < 0·001), vaginal (P < 0·01) and tumour (P < 0·001) tissues was significantly reduced. Tamoxifen therapy was without effect on the binding of [3H]oestradiol in heart tissue. The determination of specific oestrogen-binding components in vitro was significantly reduced (P < 0·01) in tumours from tamoxifen-treated rats and tamoxifen inhibited the binding of [3H]oestradiol to 8S oestrogen-binding components, derived from rat uteri and DMBA-induced tumours, in vitro.