It has been suggested that the monoclonal antibody B72.3 may be useful as a diagnostic tool in fine needle aspirates of breast masses because it recognises "tumour associated glycoprotein (TAG)-72". The antigen was sought in paraffin wax sections of 43 normal and benign breast biopsy specimens, using the avidin-biotin complex technique, to assess the extent of its presence in non-malignant tissue. Strong focal staining was seen in 21 (49%) cases. In 29 cases of fibrocystic change staining was present in 17 (59%). All areas of apocrine metaplasia were positive, as well as a few normal ducts and acini and occasional areas of adenosis. Focal positivity was present in five out of 12 foci of ductal epithelial hyperplasia and in three out of seven radial scars. Staining was absent in two areas of lobular hyperplasia, three areas of sclerosing adenosis, and in a focus of lactational change. Focal positivity was also seen in two out of five fibroadenomas and in two out of three intraduct papillomas. Five normal subareolar sections and a section of normal lactating breast were negative. It is concluded that B72.3 monoclonal antibody can show focal reactivity with a variety of normal and benign epithelial mammary structures, and it is doubtful that its use would be of any help in differentiating benign from malignant cells in fine needle aspirates.