Antibodies recognizing determinants of the alpha-subunit, which is common to all glycoprotein hormones, were eliminated from antisera against native oFSH by affinity chromatography. Since the free alpha-subunit is immunologically different from the alpha-subunit in the intact hormone, we did not use an alpha-subunit affinity column. Instead, the antisera were applied to an LH affinity column. However, because intact LH dissociates in the conditions used to elute the purified antibodies, we prepared an LH derivative with covalently-linked subunits and coupled it to gel matrix. By this method oFSH antisera were freed from their non-specific antibodies. The cross-reaction of ovine lutropin in the oFSH radioimmunoassay (RIA) was lowered from 2% to less than 0.1%. Moreover, as the columns can be used repeatedly over long periods with no apparent loss of efficiency, large volumes of antiserum can be treated in this manner.