A variety of monoclonal antibody probes to the human breast have been prepared by various groups of workers. These probes are revealing hitherto unrecognized biological and biochemical differences in both normal and neoplastic breast cells. Further study is needed, however, in order to understand fully the significance of these differences. In the meantime, the use of immune probes can improve our ability to detect disease at an earlier stage in sites such as the bone marrow, and may be heralding a new era in the detection and treatment of disseminated breast cancer.