We previously demonstrated that, in addition to the estrogen receptor, the Antiestrogen Binding Site (ABS) is also a potent mediator of the antitumorous activity of the clinical drug tamoxifen. Because of report discrepancies in the binding parameters of rat liver ABS we first attempted to improve binding study conditions. In this way buffer, protein concentration, methodology for bound/free ligand separation and phospholipidic ratio were determined. This work was used to evaluate the Stoke radius (4.4 S) and isoelectric point (pH = 6.6) of the protein in its native state. These studies constituted the obligatory transition from rat liver to pure ABS protein.