AIMS--To investigate the oestrogen and progesterone receptor concentrations expressed on mammary fibromatoses to determine their responsiveness to oestrogenic stimuli. METHODS--Six mammary fibromatoses were examined using immunohistochemistry for the presence of oestrogen and progesterone receptors using antibodies against the receptor proteins. Enzyme immunoassays (EIAs) using the same antibodies were also performed in four patients. Immunohistochemical staining for pS2 protein was also carried out as a measure of functional oestrogen receptors. RESULTS--Neither receptor nor pS2 protein was expressed using immunohistochemistry. Very low concentrations of both oestrogen and progesterone receptors were shown by EIA. CONCLUSION--These results indicate the absence of clinically important concentrations of oestrogen and progesterone receptors in breast fibromatoses and suggest that treatment directed against oestrogen is unlikely to be beneficial.