Biopsies obtained from 45 cases of invasive ductal carcinoma and 41 cases of cystic mastopathy were immunohistochemically investigated, with electron microscopy being used on some of them, for the purpose of clearing up presence, distribution, and ultrastructural peculiarities of the myoepithelial cells. Focal or diffuse myoepithelial proliferations were observed in all cases of cystic mastopathy, with peripheral layers of myoepithelium at the basal membrane being widely preserved. Myoepithelial cells had lost their normal position and cellular orientation in the regions with intraductal carcinoma. Actin-positive, high-microfilament myoepithelial cells were frequent findings recorded from parenchyma in cases of invasive ductal carcinoma without specific differentiation, though cells of that kind were rarely detectable from parenchyma in cases of medullary carcinoma. Particular attention should be given to the presence of tumor cells differing from each other for their morphological and functional differentiation in cases of carcinoma of the mammary gland.