Tenovus Institute for Cancer Research, Welsh National School of Medicine, Heath Park, Cardiff, CF4 4XX (Received 4 April 1975) Compounds having antioestrogenic properties are potentially useful for the treatment of certain patients with metastatic cancer of the breast. This report describes studies on the ability of various potential antioestrogens, in particular two nitrogen-mustard-containing substances, ICI 79792 (1-{4-β-[bis(2-chloroethyl)amino]ethoxyphenyl}-trans diphenylbut-1-ene hydrochloride) and ICI 85966 (3,4-bis{p-[(N-bis-2-chloroethyl)carbamoyl]phenyl}hex-3-ene), to prevent the uptake of [3 H] oestradiol into rat mammary tumour nuclei in vitro. Mammary tumours were induced in virgin female Sprague–Dawley rats by intubation of 7,12-dimethyibenz(a)anthracene (20 mg in 1 ml sesame oil). Tumours suitable for experimentation (2×2 cm) developed 7–12 weeks after intubation. Tumours were dissected free of connective tissue and obvious necrotic areas and minced with scissors. Equal weights of mince (1 g) were incubated (15 min at 30 °C) in Eagle's basal medium (10 ml/g tissue) supplemented with [2,4,6,7-3H] oestradiol (0·5 nmol/l,