Estrogen receptors (ERs) were assayed in 23 breast carcinomas by: (1) the conventional biochemical assay with dextran-coated charcoal (DCC); (2) the immunoenzymatic assay using a monoclonal antibody (MAb), ER-EIA (Abbott); and (3) an original cytochemical method using another MAb, ER-ICA (Abbott). The first two techniques were performed on biopsy samples, whereas the last was carried out on fine needle aspiration (FNA) samples. The ER contents in aspirates were evaluated by: (1) scaled proportions of colored neoplastic cells; (2) scaled coloration intensity; (3) total grading (= proportion plus intensity); (4) product grading (= proportion times intensity); and (5) a new index (NI) described in this paper. The ER-EIA assay correlated best, with a high statistical significance, with the NI (P less than .001); NI was also the only index that significantly correlated (P less than .05) with the DCC results. The results show that the ER-ICA assay offers the great advantages of being applicable to FNA specimens and of producing rapidly available results. This new technique enriches the panel of MAbs for the diagnosis of adenocarcinomas and offers a new tool for the therapeutic follow-up of breast cancer patients. Our preliminary results suggest that the anti-ER MAbs might be helpful for measuring the hormone dependence of small lesions not assayable by DCC, even under endocrine therapy, thus avoiding false-negative assays.