beta-Endorphin, an opioid peptide that contains 31-amino acids derived from proopiomelanocortin, has been implicated in a variety of disorders. To understand its role in pathophysiological states, its levels have been determined in body fluids, particularly in serum or plasma, with the use of commercially available radiometric assay kits. Because the circulating levels of this endogenous opioid peptide are small and antibodies can cross-react with chemically related peptides to different degrees and give rise to faulty interpretation of the data, the performance characteristics of the available radioimmunoassay kits for beta-endorphin from the Immuno Nuclear Corporation, Stillwater, Minn, the New England Nuclear Corporation, Boston, Mass, and the Nichols Institute, San Juan Capistrano, Calif, were evaluated. In overall performance, the Nichols kit was found to be the most reliable in this laboratory, even though the sensitivity of the Immuno Nuclear kit was better at lower concentrations of beta-endorphin. Serum was better than plasma in terms of recovery of beta-endorphin. When collected on ice, no significant loss in beta-endorphin immunoreactivity was observed at 1 hour. The use of edetic acid (EDTA), siliconized edetic acid, and aprotinin (Trasylol)-added siliconized edetic acid tubes was not helpful in improving the performance of the assay. The optimal condition to collect the specimen was to use glass tubes, place the glass tubes on ice immediately, separate the serum, and freeze the sample within 1 hour of collection.