Normal, dysplastic, and malignant mammary tissues are each composed of heterogeneous populations of cells. Markers that will distinguish individual mammary epithelial cells as alveolar, ductal, or myoepithelial and indicate whether a cell is normal, preneoplastic, or neoplastic represent an acute need in studies on mammary cell biology, differentiation, and tumorigenesis as well as in certain diagnostic problems of breast cancer. The proposed research will examine the expression of intermediate filament proteins in mammary cells in vivo and in vitro as possible markers with particular attention focused on the keratins. The experiments are designed to determine if ductal, alveolar, and myoepithelial cells differ in the content and organization of these cytoskeletal components and if alterations in the filaments accompany neoplastic progression. The approach will utilize biochemical and immunological methodologies including production and characterization of monoclonal antibodies. A second class of markers to be examined is phosphoproteins secreted by mouse and human mammary tumor cells. These will be studied as possible indicators of the neoplastic state and as the basis for developing a quantitative in vitro transformation assay for mammary cells. This research should not only provide new information on mammary cells of experimental and clinical importance, but also generate new reagents applicable to a variety of future investigations on all aspects of mammary cell biology.