ICI 164,384 is a novel agent, which has all the characteristics of a pure anti-oestrogen in preclinical studies. It completely inhibited oestradiol or tamoxif en-induced uterine growth in immature rats. In intact rats, ICI 164,384 produced a reduction in uterine weight, almost equivalent to that in ovariectomised rats. ICI 164,384 also demonstrated a peripherally selective action in intact rats. ICI 164,384 has a high affinity for the oestrogen receptor and was more effective than tamoxifen in blocking proliferation of MCF-7 human breast cancer cells. Extrapolation of these observations to the therapy of breast cancer would imply that a pure anti-oestrogen might provide additional benefit, compared with Nolvadex, in terms of onset, duration or completeness of response.