AbstractA modification of a conventional immunocytochemical assay for demonstration of estrogen receptors (ER-ICA) in breast cancer tissue sections resulted in a sensitive quantitative assay for identification of ER low positive subpopulations of malignant cells. The imprint technique for specimen preparation was improved, the time of antigen exposure to antibodies and DAB incubation time was extended to increase the signal for an image analysis based measurement of the intranuclear amount of peroxidase label (the ER marker). The Imagescan, a commercially available software, was upgraded to calculate this amount per single cells and identical cell populations.The ER positive human breast cancer cell line, MCF-7, was the model to test our irnmunocytochemical method against ER-ICA imprint and other immunohistochemical methods. Tumor tissue specimens from a group of breast cancer patients was used to check the diagnostic sensitivity and specificity. Data obtained by our new Quantitative Immunocytochemical ER Assay (QUICERA) were compared with the content of ER in breast tissue homogenates as measured by a conventional dextran coated charcoal binding assay, with ER positivity on frozen tissue sections assessed by the ER-ICA Monoclonal kit and the amount of the intranuclear ER positivity inside breast cancer cells freshly isolated from the same tumor tissue specimens on imprints (ERICA Imprint).QUICERA was demonstrated to be more sensitive than other methods and suitable for search of ER positive and ER negative subpopulations of cells in tumors at the borderline range of biochemical positivity. (The J Histotechnol14:191, 1991)