The purpose of this study was to determine the reactivity of monoclonal antibody (MAb) B72.3 when applied directly to aspiration biopsy cytology (ABC) of the breast in the following conditions: (1) infiltration lobular carcinoma; (2) fibrocystic disease; (3) fibroadenoma; and (4) apocrine cysts. Nine of ten aspirates from infiltrating lobular carcinoma were positive in these assays, while 21 of 22 benign cases reacted negatively. The single false-positive benign aspirate manifested a staining pattern characteristic of apocrine cells. This study demonstrates that the MAb B72.3 can be employed as a potentially valuable diagnostic adjunct. It can be used on stained aspirates to assist in the interpretation of ABC from breast lesions.