Specific estradiol receptors in cytosols from human breast tumors were measured by discontinuous polyacrylamide gel electrophoresis. 2. Binding of estradiol to contaminating sex hormone binding globulin was clearly separated from that due to the specific receptor. 3. The amount of estradiol bound by the receptor was linear with respect to the amount of protein added to gel. 4. The estradiol receptor peak of radioactivity showed saturability and specificity characteristic of an intracellular receptor. 5. The results show that polyacrylamide gel electrophoresis can be used to quantitate specific estradiol receptors in the presence of other high affinity binding components.
