Myoepithelial cells are normally located between the epithelial cells and the basal lamina of secretory elements of exocrine glands. Their role in the histogenesis of breast tumours has been studied extensively, and a definite differentiation towards myoepithelial cells has been demonstrated in adenoid cystic carcinoma, adenomyoepithelioma, low-grade adenosquamous (syringomatous) carcinoma, pure malignant myoepithelioma and poorly differentiated myoepithelial-rich breast carcinoma. All these tumours are of low malignancy, with the exception of malignant myoepithelioma and poorly differentiated myoepithelial-rich carcinoma. We examined the possibility that invasive ductal carcinoma of the breast might show differentiation towards both epithelial and myoepithelial cells because there is no reason to assume that one type of differentiation necessarily excludes the other. We performed the avidin-biotin immunohistochemical analysis of 20 cases of infiltrating ductal carcinomas (IDCs) with diffuse fibrosis, 20 cases of IDCs without fibrosis and five cases of metaplastic carcinomas, to detect myoepithelial differentiation of the tumour cells. Myoepithelial differentiation was determined by the expression of alpha-smooth muscle actin (alpha-SMA). We concluded that IDCs with diffuse fibrosis are associated with a myoepithelial immunophenotype of carcinoma cells.