Immunoglobulin from the serum of rabbits immunized with highly purified estradiol-receptor complex from calf uterine nuclei has been shown to contain specific antibodies to estrophilin by five criteria. Antibodies to calf nuclear estrophilin cross react with nuclear estradiol-receptor complexes of rat, rabbit and sheep uterus, rat endometrial and pituitary tumor, and MCF-7 human breast cancer cell line. They also react with extranuclear receptor of calf, rat, mouse, rabbit, guinea pig, monkey and sheep uterus, rat mammary, endometrial and pituitary tumor, and human breast cancer. There is no interaction of the antibody with estradiol itself. The nuclear form of estrophilin appears to bind more immunoglobulin molecules than does the cytosol form. The antibodies do not react with either the nuclear or extranuclear dihydrotestosterone-receptor complexes of rat prostate, with the extranuclear progesterone-receptor complexes of rabbit uterus, chick oviduct or rat endometrial tumor, or with rat and mouse alpha-fetoprotein. These findings indicate an immunochemical similarity among estrophilins from several mammalian species, as well as between nuclear and extranuclear forms of the receptor, but not among receptor proteins for different steroid hormones. Immunoglobulin from the serum of a goat immunized with similar antigen shows a considerably higher titer of antibodies to estrophilin. These react with nuclear and extranuclear estradiol-receptor complexes of calf uterus to produce somewhat larger entities than those formed with the rabbit antibody. Unlike the rabbit antibody, interaction with the goat antibody causes a noticeable decrease in estradiol-binding affinity of the extranuclear estrophilin as well as an apparent decrease in the total hormone-binding capacity. Specific antibodies to estrophilin offer promise as valuable reagents for receptor analysis and purification, as well as for the elucidation of many still unresolved questions concerning receptor synthesis, localization and function.