We have investigated the effects on breast cancer cell growth of 4-hydroxytamoxifen (4OHT), a conventional antioestrogen with agonist activity, and 7 alpha-[9-(4,4,5,5,5-pentafluoropentylsulphinyl)nonyl]oestra- 1,3,5,(10)- triene-3,17 beta-diol (ICI 182780), a novel, pure antioestrogen, using established human breast cancer cell lines and cancer cells obtained directly from breast cancer patients with malignant pleural effusions who had relapsed on tamoxifen. The effects of the two agents were assessed using the Courtenay-Mills clonogenic assay, which measures the growth of single cancer cells as colonies suspended in soft agar. The standard assay was modified by the use of defined serum- and phenol red-free growth medium. The growth of oestrogen receptor (ER)-positive MCF-7 cells in the assay was oestrogen responsive. Both antioestrogens inhibited the stimulatory effects of 1 nM oestradiol, but ICI 182780 caused significantly greater inhibition than 4OHT at 0.1-1.0 microM concentrations. In the absence of oestradiol, 4OHT but not ICI 182780 caused significant stimulation of colony formation at low (0.01-1.00 nM) concentrations. Neither antioestrogen had any effects on colony formation by the ER-negative Hs578T cell line. Successful colony formation was obtained in primary cultures from six out of eight malignant effusions. Colony formation was significantly stimulated by 0.1 nM oestradiol in four cases and by 10 nM 40HT in two cases. In contrast, ICI 182780 exhibited no intrinsic stimulatory activity and significantly inhibited both oestradiol- and 4OHT-stimulated cell growth. We conclude that the agonist activity of 4OHT and other conventional antioestrogens may cause treatment failure in some patients by stimulating breast cancer cell growth. The new, pure antioestrogen ICI 182780 is a more potent oestrogen antagonist than 4OHT and exhibits no growth-stimulatory activity. This agent may therefore offer therapeutic advantages over conventional antioestrogens in patients with advanced breast cancer and may be effective after conventional agents have failed.