Our knowledge of the biological role of the antiestrogen binding site ABS in the antitumoral activity of tamoxifen, will be increased with the determination of its coding gene sequence. To this end our team has for some time attempted to purify this membranous protein. In this work we report the purification to homogeneity of ABS from rat liver in a six step succession. Specific photolabeling with a tritiated photoprobe, solubilization of rat liver microsomes, chromatofocusing of the labeled proteins, preparative electrophoresis on polyacrylamide gel, and two consecutive high performance liquid chromatography separations on C4 hydrophobic resin produced 2.5 micrograms of pure ABS by silver stain analysis of SDS-PAGE. The NH2-terminal residue of the protein appears to be blocked, which hinders the Edman degradation method for obtention of the whole protein sequence.