Cytosols from human breast carcinomas rich in estrogen receptors (ER) were examined for the presence of [3H]-estradiol (E2) and [3H]-monohydroxytamoxifen (OH-TX) binding components. Polyacrylamide gel electrophoresis was used to examine the comparative anodal mobilities of ER-[3H]-E2 and ER-[3H]-OH-TX complexes, and also to identify any cytosol or serum component that may exhibit preferential high affinity to OH-TX. We have demonstrated that [3H]-OH-TX binds ER with high affinity and the anodal mobility of ER-[3H]-OH-TX complexes is identical to that of ER-[3H]-E2 complexes. We were unable to identify an antiestrogen-specific component in ER-positive or ER-negative cytosols or in sera of healthy adult females. A serum component exhibiting a higher affinity to [3H]-OH-TX and [3H]-DES than to [3H]-E2 has been identified in all the female sera examined, but this binding is of high capacity and is unsaturable by a 1000-fold molar excess of unlabeled E2 or antiestrogens. The electrophoretic mobility of this component is comparable to that of serum albumin.