Methods for the measurement of cytoplasmic and nuclear oestrogen receptors (CER and NER) are described. CERs and NERs were found to be labile at ambient temperatures; the receptors are stable for up to 1 year when stored in liquid nitrogen. Over a period of 5 years, 51% of the breast adenocarcinomas examined in women proved to be positive for CER; four adenocarcinomas of the breast obtained from men were all positive for CER; in those tumours examined for both CER and NER, 46% proved positive for CER and 30% positive for NER; no tumour was found to be positive for NER and negative for CER. In older female patients, the percentage of patients positive for oestrogen receptors increased and they were present in higher concentrations; in younger Indian women the same tendency was observed. The percentages of receptor-positive tumours in Indians, Blacks and Whites are similar. Neither the stage of the tumour nor the presence of nodal metastases influenced receptor positivity; there was no significant difference in the time to recurrence between receptor-positive and receptor-negative patients in the small number with reliable follow-up data.