Fine needle aspirations of breast masses were evaluated to establish a cytologic diagnosis. In 45 cases, the sensitivity was 73%, specificity 100%, and diagnostic accuracy 84%. Fifty cases (27 malignant and 23 benign) were evaluated by immunoperoxidase staining of the needle aspirates, using the monoclonal antibody B72.3. Twenty-six of 27 malignant cases stained positively, and 22 of 23 benign cases were negative. The sensitivity was 96%, specificity was 96%, and diagnostic accuracy was 96%. All cases considered malignant cytologically were confirmed by immunocytochemistry. In addition, the B72.3 monoclonal antibody appears to be a valuable diagnostic adjunct in atypical and suspicious cases, particularly those in which hypocellularity and/or cellular monomorphism preclude a diagnosis of malignancy by routine cytologic examination.