In the present study the authors have examined 100 breast nodules by fine-needle aspiration cytology (FNAC) using the avidin-biotin complex method of immunoperoxidase staining. The monoclonal antibodies (MAbs) employed, B 6.2 and B 72.3, recognize two distinct molecules expressed by transformed breast epithelium. The authors have compared this immunocytochemical reaction with the conventional cytology and the histological examination as well. The data of the MAbs alone or in combination have been evaluated in terms of sensitivity, specificity, predictive value and test efficiency. The results show that tumor cells are strongly reactive with the MAbs. So the authors believe that this method in conjunction with conventional cytology may be useful in the diagnosis of breast tumor, especially in doubtful FNAC.