AbstractMeasurement of estrogen and progesterone receptors (ER and PR) is very important in selecting treatment for patients with breast cancer because some tumors respond to hormonal treatment. The standard methods for receptor measurements have been the steroid binding assays, especially the biochemical cytosol. These assays have several limitations, among which are that they can not identify tumor cells and they can not be used with routinely fixed and processed tissue. Immunocytochemical techniques with monoclonal antibodies are compatible with the study of receptor content in malignant cells, which is believed to be a good prognosticator of long term survival. One commercial estrogen receptor-immunocytochemical assay kit (ER-ICA) has received Food and Drug Administration approval for diagnostic use; however, it was designed for use on fresh frozen, mildly fixed sections.This study examined the parameters that would permit successful immunostaining with the ER-ICA in routine histologic sections: enzyme digestion, repeated application of primary antibody, and imidazole in the color development step. Results from ER-ICA performed in paraffin correlated well with those from the cytosol assay, and the technique could be of great value in cases where there is insufficient material for biochemical assay or when frozen material is unavailable. (The J Histotechno1 13:29, 1990)