The pNR-2 mRNA is regulated by oestrogens in cell lines established from metastatic human breast cancer cells. The levels of the pNR-2 and oestrogen receptor RNAs have been measured in 96 tumour samples from patients undergoing surgery for breast cancer. Oestrogen receptor mRNA was detected in 90% of the 60 primary breast tumour samples from patients not receiving endocrine therapy at the time of surgery, whereas the pNR-2 RNA was detected in 57%. In primary tumours the expression of pNR-2 was entirely dependent upon oestrogen receptor RNA expression. When the 60 primary tumours were considered, pNR-2 and oestrogen receptor mRNA levels were significantly correlated. There was no significant correlation for pNR-2 positive tumours. pNR-2 mRNA levels were similar in tumours of pre- and post-menopausal patients and were independent of tumour differentiation and nodal status. Oestrogen receptor and pNR-2 mRNA levels were also measured in 21 tumour samples from patients receiving primary tamoxifen therapy. Eleven of these had shown an objective response and a significantly larger number of tumours from these patients contained pNR-2 mRNA than from patients who did not respond (chi 2 = 6.08, P less than 0.025).