NCRC 11 is a monoclonal antibody raised against dissociated breast cancer cells. Using this antibody, it has been shown in an earlier study that staining of a high proportion of tumour cells is strongly associated with superior survival. Many investigators have found that positive tumour oestrogen receptor (OER) status is associated with better survival in breast cancer. In the present study of 136 breast cancer patients followed up for a minimum of 30 months, tumour oestrogen receptor assays were carried out, and using the indirect immunoperoxidase technique, histological sections of paraffin embedded material were stained for NCRC 11; for the purpose of this study, tumours showing 25 per cent or less cells staining were regarded as 'negative'. Patients whose tumours were OER positive had a significantly better prognosis (logrank test P less than 0.05). Patients whose tumours were graded as negative for NCRC 11 had a poorer prognosis compared with the positive group but this did not attain significance. Our failure to demonstrate convincingly a better prognosis for the NCRC 11 positive patients may relate to the shorter duration of our study, or to different tissue fixation techniques. In this study staining for NCRC 11 was positively correlated with oestrogen receptor status (P less than 0.02).