Estrogen receptors (ER) were determined by both the biochemical dextran-coated charcoal (DCC-ER) and the immunohistochemical Avidin biotin-peroxidase complex (IHC-ER) methods in proliferative mammary lesions collected from 37 cats: 20 malignant tumors without metastasis at first presentation, seven malignant tumors with lung and/or lymph node metastases and 10 benign tumors and dysplasias. Total number of samples analyzed by both methods was 44. The DCC-ER method was applied to frozen tissue samples and the IHC-ER method was applied to neutral buffered formalin-fixed, paraffin wax-embedded tissue samples by using the NCL-6F11 monoclonal antibody. Biochemically, 21 (47.7%) cases had equal or more than 5 fmol/mg of protein (standard positivity threshold). Immunohistochemically, 11 (25%) cases were scored positive, the percentage of positive nuclei being statistically linked to the intensity of immunostaining. Normal mammary gland tissue (13 cases) and/or dysplastic areas (5 cases) found in the surroundings of the main lesion were IHC-ER positive in 76.9% and 40% of the cases, respectively. Concordance between DCC-ER and IHC-DCC was 72.7% and the results of the DCC and the IHC-ER methods were significantly correlated (P < 0.05) by the chi2 test. Specificity (true negatives) and sensitivity (true positives) of the ICH-ER method were 95.6% and 47.6%, respectively. One out of eleven DCC-ER positive and IHC-ER negative discordant cases (9.09%) was a DCC-ER false positive, because the surrounding normal mammary gland tissue was IHC-ER positive. The remaining 10 cases had ER content values equal or lower than 23 fmol/mg of protein, a figure that could represent the sensitivity threshold of the immunohistochemical method employed.