Nuclear estrogen receptors (ERN) were measured in human breast cancer and 7,12-dimethylbenz[a]anthracene-induced mammary carcinoma using the dextran-coated charcoal method without addition of an adsorbing material such as hydroxyapatite. Cytoplasmic estrogen receptors (ERC), cytoplasmic progesterone receptors (PgR), and ERN were measured in 111 primary and 11 locally recurrent breast cancers. ERC were found in 52%, PgR in 35%, and ERN in 40%. ERN and PgR were more frequent in tumors with a higher level of ERC. PgR were more frequent in ERN-positive tumors, and ERN were more frequent in PgR-positive tumors. The addition of an ERN assay to both ERC and PgR assay may be more informative for prediction of estrogen dependency than the mere determination of the existence of receptors in the cytoplasm.