Detection of tumor-markers released into the general circulation indicates tumor presence and also provides information on the antigenic make-up of the tumor that released them. The study of cell tumor-markers in leukemia, where usually neoplastic cells are readily available, has been very fast. In contrast research on those tumor-markers associated with carcinomas has been much slower, mainly due to the fact that tumor tissue is usually available only once, at the time of surgery. In addition, the separation of the neoplastic cells from the tissue mass is difficult and their survival in vitro not always satisfactory. Presently, we have developed very sensitive and specific techniques for breast tumor-marker identification and extraction from serum. With the aid of these techniques, that employ polycolonal and monoclonal antibodies to accomplish the separation of the breast differentiation tumor-markers from serum, we plan to: a) extract and characterize these tumor-markers from the sera of patients with breast tumors. Of special interest will be those tumor-markers derived from the cell membrane, however, other markers will be considered; b) study the fate of breast tumor-makers in the circulation (their binding properties, catabolic destruction, organ of excretion, etc.); and c) investigate how these breast tumor-markers are released from the tumor cells and how they gain access to the circulation. The development of methodology for the appropriate identification of breast tumor-markers in circulation could make it possible to characterize a tumor mass in terms of its antigenic make-up by just a serum sample, thus being able to provide indications on tumor antigenicity and progression that could be of therapeutic and/or prognostic value. In addition, our understanding of how these tumor components are carried in circulation, and how they are disposed of could provide information valuable to assess the patient's immune status. Finally, the study of how these markers are released from tumors could give us clues of how the tumor interacts with the host at the cellular level.