Abstract The ability to detect oestrogen receptor in human tumour cytosols can be of value in selecting between treatment options and in determining prognosis. Assay of this receptor is dependent upon the specific binding of radiolabelled oestradiol. Therefore the development of methods, which minimise the loss of integrity of the oestradiol binding site during cytosol preparation and receptor assay, is desirable. The data presented in this communication show that sodium molybdate stabilises the oestrogen receptor isolated from human myometrium and MCF 7 human breast cancer cells. The stabilisation of the receptor was observed both qualitatively, when the 8S form of the receptor was observed in sucrose density gradients only in the presence of molybdate, and quantitatively, when the presence of molybdate was accompanied, in all cases examined, by a significant increase in the concentration of oestrogen receptor measured.