Recent reports suggest that oestrogen receptors detected in the cytosolic fraction of homogenized human breast tumours might be mainly nuclear receptor released into the cytoplasm during tissue processing. This study thus compares the tumour content of steroid hormone receptors in conventional cytosolic receptor assays with direct measurements of receptor in the cell nucleus. Unoccupied cytoplasmic oestrogen receptors (cER), cytoplasmic progesterone receptors (cPR) and total (occupied plus unoccupied) nuclear oestrogen receptors (nER) have been measured in parallel in human breast tumour tissue using biochemical radioreceptor assay. Of 125 tumours, 62% and 61% were positive for cER and cPR, respectively, 50% contained nER with high affinity for oestradiol (nER I) and 13% expressed nER with low affinity for oestradiol (nER II). The concentration of cER correlated significantly with age, cPR and log nER I. A significant proportion of tumours which were positive for both cytosol receptors also possessed nuclear receptors with high and low affinity for oestrogen. It is possible that this group of tumours which are positive for cER, cPR, nER I and nER II will respond well to hormone therapy.