Gross cystic disease fluid protein-15 (GCDFP-15) is a commonly used apocrine marker; however, its expression was recently found to decrease in infiltrating, larger, or metastasizing apocrine carcinomas of the breast. In the breast, monoclonal antibody (MAb) B72.3 has been reported to be useful as an apocrine marker although it is used for that purpose much less frequently than GCDFP-15. In the search for a more consistent apocrine marker, immunoreactivity for MAb B72.3 was examined in apocrine carcinomas at different stages and compared with GCDFP-15. 47 of 51 apocrine carcinomas (92%) and 9 of 62 ordinary carcinomas (15%) were MAb B72.3 positive, while 39 of 51 apocrine carcinomas (76%) and 13 of 62 ordinary carcinomas (21%) were GCDFP-15 positive. Thus, both sensitivity and specificity were higher for MAb B72.3. Furthermore, unlike GCDFP-15, MAb B72.3 exhibited positivity irrespective of infiltrating status, tumor size, or metastatic status. There was no correlation between MAb B72.3-immunoreactivity and GCDFP-15-expression. The combined usage of MAb B72.3 with GCDFP-15 was useful to confirm the diagnosis of apocrine carcinoma, especially for advanced tumors, with only two cases being negative for both MAb B72.3 and GCDFP-15. Whether these two cases should be differentiated from ordinary apocrine carcinomas remains to be investigated.