The heterogeneity of oestrogen receptor (ER) expression has been examined in both normal and malignant breast tissue using an immunohistochemical assay. In both instances the ER status and cellular ER negativity were influenced by the patients' menopausal status, with tissues removed from premenopausal women being more often ER-negative, and when ER-positive, containing a high proportion of apparently ER-negative cells. Since the breast is normally regarded as hormone sensitive and since tumour cell ER negativity is apparently under a degree of hormonal regulation, our results suggest that the proportion of breast cancer cells that are ER-negative should be viewed with a degree of caution.