A malignant myoepithelioma, mainly intraductally growing, of the breast was studied, emphasizing the immunohistological features of such tumors, in comparison with the more common, architecturally similar, infiltrating ductal carcinoma with intraductal component. Immunohistochemically, cell population of the tumor studied expressed intense reaction to alpha-SM-actin, cytokeratin 14, vimentin, S-100 protein and collagen IV antibodies, confirming myoepithelial differentiation. Since myoepitheliomas of the breast are very rare, complete knowledge of these neoplasms has not been attained. Nevertheless, histogenetic investigations may be useful in differentiating this type of tumor, from the above one, which is known to be characterized by severe clinical and prognostic implications.