A method aimed at the identification and quantitation of estradiol receptors has been developed. The test is based on competitive binding reaction between solubilized cell receptors and antiestradiol antibodies bound to acrylamide beads, for ³H-estradiol. The technique can be employed both for estradiol radioimmunoassay and quantitation of specific receptors in tissues and appears to be a sensitive, specific and reproducible method which can be applied to the evaluation of any hormone and its receptor.