1. Immunoradiometric assay crossreactivity between human LH and human FSH was used as a model system in order to develop and evaluate methods for the isolation and use of any specific high-affinity antibodies present in nonspecific antisera. 2. The purified radioactive antibody reagent was treated quantitatively with immunoadsorbents containing the cross-reacting or contaminating antigen to yield an assay of improved specificity but reduced precision. 3. The “treated” antibody reagent was then repurified to discard antibodies of low affinity and antibodies which dissociated rapidly from the antibody/antigen complex. This produced a specific, high-affinity antibody reagent and a specific assay of improved precision. 4. Also studied were: (a) variations in the dose-response curve caused by prolonging the second assay reaction, (b) an artificially “contaminated” assay system which measured both LH and insulin.