The aim of this proposal is to prepare monoclonal antibodies against the human breast cancer cell protein 24k. The antibodies will be prepared by the methodology of Kohler and Milstein and be used to establish a radioimmunoassay for 24k in breast cancer biopsies. A study will be done to compare estrogen and progesterone receptor with 24k in predicting responsiveness to endocrine therapy of breast cancer. The antibodies will also be used to select subsets of breast cancer tissue culture cells particularly responsive to estradiol.