This study deals with the pattern of estrogen receptors in 52 cases of invasive breast carcinoma by comparing the ligand-binding assays with dextran-coated charcoal (DCC) to an immunoperoxidase assay (IPE) technique in paraffin-embedded material from the same tumor. A modification of the IPE by adding cobalt chloride in the final reaction and counter-staining with eosin instead of hematoxylin was introduced. A significantly high correlation was found between the two compared methods (80.8% positivity by the DCC method and 80% by the IPE method). The correlation was very significant in cases where the immunohistochemical grading was higher than 4 (according to our semi-quantitative evaluation scale of 0-8), and for these cases we suggest the IPE method as highly specific and precise.