Erosive adenomatosis of the nipple (EAN) is a complex benign mammary proliferation that has a variety of histologic appearances. All forms of this lesion are thought to be composed of two apparent cell types: epithelial luminal cells and basal myoepithelial cells. A panel of immunohistochemical reagents was employed to determine whether this divergence of differentiation could be confirmed immunohistochemically. It included antibodies against cytokeratin (CK), vimentin (VIM), glial fibrillary acidic protein (GFAP), and muscle-specific actin (MSA); the latter three markers are associated with myoepithelial differentiation. EAN was also assessed with reagents directed at carcinoembryonic antigen (CEA) and collagen type IV (CIV). Basal glandular cells in EAN homogeneously expressed CK and MSA. They also were positive for VIM in 10 cases, and for GFAP in three. Luminal glandular cells in EAN expressed CK uniformly and CEA focally. Lesional glands were surrounded by a concentric layer of CIV. The authors conclude that two cell types are indeed demonstrable immunohistochemically in EAN, one of which has myoepithelial features. CEA may be expressed by this lesion despite its innocuous biological nature, and constituent glands synthesize basement membrane proteins in a pattern that is characteristic of benign mammary proliferations. Differential diagnosis between EAN and invasive carcinoma of the breast may be facilitated by immunohistologic evaluation, but the latter modality of study is not capable of distinguishing between EAN and other benign proliferative mammary lesions.