The distribution of simple epithelial (K8/18/19) and basal (myoepithelial) (K5/14) keratins, alpha-smooth-muscle actin, vimentin, collagen IV and laminin in normal mammary glands and in benign proliferative lesions was studied using monoclonal antibodies (mAbs). These antibodies (Abs) identified myoepithelial cells and luminal cells specifically. In lesions with adenosis and papillomas, the two-layered formation resembled that of normal glands with a purely myoepithelial-epithelial differentiation. In scleradenotic lesions, the main cell was of myoepithelial immunophenotype with intermixed trabecular-tubular proliferations of simple-type epithelium. The sclerosis seems to be the result of an irregular basal lamina synthesis by the myoepithelial cells. In contrast to these lesions, epitheliosis represents a purely intraluminal cell proliferation of clearly simple epithelial immunophenotype and of cells with a basal keratin phenotype, lacking myoepithelial differentiation antigen actin. The basal keratin type epithelium may represent post-stem or intermediate cells developing into luminal epithelium. Epitheliosis appears to be a purely epithelial hyperplasia with striking similarity to the regeneration of normal breast epithelium. The different proliferative patterns may give an explanation for differences in potential cancer risks of patients with these lesions.