Actomyosin-containing cells in both non-neoplastic and neoplastic tissues of the salivary gland, lung, breast and some other organs were studied by immunofluorescent microscopy using antiactomyosin rabbit serum. In the breast, myoepithelial-like cells with positive immunofluorescence in the cytoplasm were observed not only in sclerosing adenosis and fibroadenoma but also in scirrhous and medullary-tubular duct carcinomas. No positive cells were observed in medullary carcinomas with lymphoid infiltration. The actomyosin positive cells were also seen at the outer layer of tubules of "mixed tumors" and of cell nests in adenoid cystic carcinoma and in myoepithelioma of the salivary gland, but not in the metaplastic squamous cells or in the cells of myxomatous and chondroid areas of "mixed tumor". In carcinoma of the lung, actomyosin-positive cells were observed in adenoid cystic carcinomas and adenocarcinoma of the bronchial gland type, but they were not seen in squamous cell carcinomas or papillary adenocarcinomas. It was concluded that the actomysoin-containing cells with structural appearances of myoepithelial cells in a variety of tumors were neoplastic myoepithelial cells.