The wealth of research results in the area of breast cancer diagnosis and therapy with monoclonal antibodies presented in previous workshops is now complemented with a new rendition of the proceedings of the 4th Workshop on Monoclonal Antibodies and Breast Cancer. held in San Francisco on November 5-6. 1990. Basic science findings reported in previous workshops have now percolated to the clinical level and have become immunoassays and imaging and therapy reagents. as the program shows us. Thus. the latest discoveries in immunology. biochemistry and molecular biology of breast epithelial antigens and their corresponding antibodies have produced newer diagnostic tests and therapeutic approaches that are altering and improving the way we attack breast cancer. The recent spectacular and rapid advancements in the molecular biology of several of the breast epithelial antigens are presented in this volume. The way in which the final assembly of different components of the breast antigens is achieved and their functions are now within our grasp as a result of new understanding of molecular structure of these breast antigens. In addition. newer immunoassays aiming at the earliest detection of the disease are also described that integrate with promising attempts at imaging and radioimmunotherapy to set the stage for new oncological possibilities in breast cancer treatment. All these areas of intense involvement of scientists with diverse specialties are presented in this volume. which proves the need for multidisciplinary approaches to increase our chances for success in this field of medical research.