The localization of estrogen receptors (ER) and progesterone receptors (PgR) in human breast cancer was examined light and electron microscopically by using specific monoclonal antibodies. Among the breast cancer patients, ER-positive (ER[+]) cases were shown by enzyme immunoassay (EIA) to account for 58.9% (63/107), and 63.5% of these cases were women in the postmenopausal state. The PgR-positive (PgR[+]) rate was 50.5% (54/107), with 44.4% of these positive cases being postmenopausal women. The rates of ER(+) and PgR(+), ER negative (ER[-]) and PgR negative (PgR[-]), and ER(+) and PgR(-) cases were 43.9% (47/107), 34.6% (37/107) and 15.0% (16/107), respectively. Although ER(-) and PgR(+) cases were in few number, they were found (6.5%, 7/107). These results correlated well with those obtained by the immunocytochemical method. In either case, i.e anti-ER or anti-PgR reaction, positive nuclei and negative nuclei were found intermingled with each other in a given visual field. In electron microscopy, both anti-ER and anti-PgR antibodies bound to sites in the euchromatin area of the nucleus.