A high-molecular-weight protein with beta-endorphin- and adrenocorticotropin-immunoreactivities was isolated from extracts of human placenta after several purification steps, including immunoadsorption with a well-characterized antiserum raised to beta-endorphin. This protein was identified as the heavy chain of the human immunoglobulin class IgG1. These results have led to the recognition of homologies in the amino acid sequences of these physiologically unrelated molecules. They also suggest caution in accepting immunological competence as the sole criterion of the chemical identity of a ligand.