Antiestradiol antisera were raised in rabbits by immunization with a steroid-protein conjugate Whole antisera and purified antiestradiol antibodies were reacted with heat-killed and formalin-fixed bacteria from the protein-A-bearing strain of Staphilococcus aureus (Cowan I, NCTC 8530). The bacterial immunoadsorbent was used to estimate the estrogen-receptor proteins contained in several specimens from human breast carcinoma. Since the affinity constant for estradiol of antiestradiol antibodies is about 10(8) M-1 while those for breast estrogen-receptor complexes are about 10(9) to 10(10) M-1, it is possible to remove the free steroid from a reaction mixture containing tissue cytosol, radio-labelled estradiol, and the antiestradiol bacterial adsorbent by pelleting the bacteria at low-speed centrifugation. The radiolabelled estradiol molecules bound to their receptors remain in the supernatant and can be easily counted by liquid scintillation counting. The results obtained by this technique on specimens of human breast carcinomas compared more than satisfactorily with those obtained, on separated aliquots from the same specmens, by the dextran-coated charcoal method, according to the EORTC group (1973). The versatility, specificity and stability of the antiestradiol bacterial immunoadsorbent recommend its use, rather than that of charcoal-coated dextran and other non-specific steroid adsorbents, in the mass screening of patients with breast carcinoma which could be amenable to hormonal therapy.