Evaluation of estrogen receptors (ERs) is essential for the treatment and prognosis of human breast cancer. The authors have undertaken a study of 225 primary breast cancers to compare the receptor values using immunohistochemistry and enzyme immunoassay (EIA) techniques. The results are compared with the biochemical assay with the use of the dextran-coated charcoal (DCC) method done on the same specimens. The overall concordance between ER-ICA and DCC was 77%. Ninety-seven percent concordance was observed for specimens with a positive ER-ICA score (greater than 70 and the DCC more than 10 fmol/mg protein). Ninety-two percent of tumors had negative results by ER-ICA when the DCC was less than 10 fmol/mg protein. When the ER-ICA score was plotted against the DCC or EIA value, a significant correlation was obtained with a P value of less than 0.001. The correlation between the ER-ICA score and the total ER content (cytosol and nuclear) assayed by EIA was similar to comparison with cytosol alone. The authors conclude that the ER-ICA can be useful in assessing ERs in breast especially in hypocellular or small tumors and in assessing the degree of heterogeneity in a given tumor. The immunohistochemical and biochemical assay methods are complementary and provide greatly needed information for the management of breast cancer.