Anti-idiotypic antibodies to anti-leucine enkephalin raised in rabbits immunized with leucine enkephalin conjugated to BSA, were purified and characterized for their ability to mimic leucine enkephalin in binding to the rat brain delta opioid receptor. An ELISA was standardized to assay the delta opioid receptor using these antibodies. The rat brain receptor was purified to 308 fold with a yield of 1.5% using these antibodies for assay. The receptor, probed with these anti-idiotypic antibodies, was characterized with respect to binding parameters, molecular weight and effect of divalent cations.