A monoclonal antibody prepared against estrogen receptor has been shown to be highly specific and sensitive for the detection and quantification of estrogen receptor in human breast lesions using immunohistochemical methods. A semiquantitative relationship has been shown between the intensity of staining and biochemical receptor analysis. To evaluate the usefulness of this technique in fine needle aspiration biopsies, 41 cases of breast cancer were studied. Nuclear localization of receptor antibody PAP complex was observed. Comparison of the immunocytochemical analysis of the aspiration cytologic specimens to the biochemical analysis of the excised tumor mass revealed a sensitivity and specificity of 80% and 89%, respectively. There was a significant semiquantitative correlation between the methods. These results extend the observation that antireceptor monoclonal antibody in immunohistochemical analysis is an effective tool in the evaluation of estrogen receptor content in human breast lesions and with careful specimen handling and proper controls may be extended to the evaluation of fine needle aspiration biopsies.