The objective of this study was to determine the reliability of immunocytochemical assays of hormone receptors using specimens obtained by fine-needle aspiration of breast cancers. A peroxidase-antiperoxidase immunocytochemical assay was used on 96 aspirates from 47 patients to determine estrogen and progesterone receptor status. Of 27 estrogen receptor-positive cases by steroid-binding analysis, 25 were positive by immunocytochemical assay. Of 21 estrogen receptor-negative cases, 17 were negative by immunocytochemical assay. Of 21 progesterone receptor-positive cases 19 were positive by immunocytochemical assay. Of 27 progesterone receptor-negative cases 25 were negative by immunocytochemical assay. An additional 11 small tumors were evaluated by immunocytochemical assay alone and results were interpretable based on our prior data. With the caveat that a cellular aspirate is obtained, immunocytochemical assay can distinguish hormone receptor-negative and hormone receptor-positive tumors and can be used to assay tumors too small for standard steroid-binding analysis.