Eleven samples from 7 primary and 4 metastatic male breast cancers were analysed for cytoplasmic oestrogen receptors. Two samples of metastatic cancers were obtained from a same patient 6 months apart. Receptors were assayed by measuring the binding affinity of cytosol fractions for 3H-oestradiol- 17beta. Saturable binding sites were obtained in 6 primary and 1 metastatic cancers. The dissociation constant of the binding reactions were within the range reported for the specific oestrogen receptors from female breast cancers. When present, receptor concentrations varied from 59 to 532 fmoles/mg tissue protein. Competition studies of the binding of 3H-oestradiol- 17beta by excess amounts of unlabelled steroid hormones or antihormones revealed that the receptors were specific for oestrogens and antioestrogens. By all these criteria, receptors from male mammary cancer were identical to those from female breast cancer. Chromosome analysis was performed in 6 patients. All had normal 46 XY male pattern indicating that no conspicuous genetic abnormalities such as those of the Klinefelter syndrome seemed responsible for the presence of the receptors.