Correlations between the oestrogen receptor (ER) content and histology of human breast cancer are reviewed. Although conventional receptor assays of tissue homogenates may give misleading results because of tumour heterogeneity, certain trends are apparent: lobular carcinomas are rather more commonly ER positive than are ductal carcinomas; cellularity, differentiation, elastosis, and paucity of stromal lymphocytes are positively correlated with ER content in ductal carcinomas. Attempts to detect and localize steroid hormone binding sites in breast cancer cells or sections by histochemical or immunohistochemical methods do not satisfy the criteria for demonstrating classical high affinity cytosolic receptors. Nevertheless, the clinical relevance of demonstrating binding sites of lower affinity may be worthy of further investigation. Breast cancer is an ideal model for exploring correlations between structure and function in tumours.