The effect of systemic therapy on tumour oestrogen receptor (ER) concentration has been studied in 88 patients with large, operable, primary tumours (total 89) of the breast. In 26 patients, tumour was not available for study on one occasion (usually post-treatment). Forty-five patients were treated initially by endocrine therapy but, of these, 13 who had failed to respond went on to receive chemotherapy also. Seventeen patients with low concentrations of ER (less than 20 fmol mg-1 protein) were treated directly by chemotherapy. Patients underwent an incisional biopsy for confirmation of diagnosis and determination of pre-treatment ER by radioligand binding assay, followed by systemic therapy for 3 months (or 6 months for both endocrine and cytotoxic therapies). Response was assessed clinically and mammographically before mastectomy. ER concentration was then determined in the post-treatment tumour specimen. No significant change in ER concentration was seen in any treatment group except when the patients had received tamoxifen; there, receptor concentration fell to very low levels, presumably due to interference with the assay. There was no relationship between tumour response to systemic treatment and change in ER concentration. It is concluded that changes in ER concentration are unlikely to play a major role in the early response of breast tumours to systemic therapy.