By application of a monoclonal antibody (Abbott ER-ICA) oestrogen receptors can now be demonstrated and semiquantitated on paraffin sections after fixation in formalin. In a prospective series of 22 breast carcinomas we found that 82% (18/22) were oestrogen receptor positive. The frequency was significantly lower in archival tissue comprising 27 invasive carcinomas (56%) and in 63 intraductal carcinomas (54%). Even lower frequencies were found in recurrences and metastases and none (0/10) of the metastases from tumours of unknown origin were oestrogen receptor positive. It is likely that prolonged fixation in formalin destroys the receptor protein. Immunocytochemical assessment of oestrogen receptor may be the only alternative when dealing with small breast tumours (< 1 cm), in cases of suspected recurrences and in cases of unknown receptor status.