Enkephalins, derived from large precursors pro-enkephalin A and B, are abundant opioid peptides found extensively in the brain, peripheral nervous system, and adrenal medulla. Various neuroendocrine tumors, such as pheochromocytomas, neuroblastomas, and bronchial/gastrointestinal endocrine tumors, produce enkephalin. Absorption with 10-100 ug immunogen per ml diluted antiserum eliminates staining, unaffected by beta-endorphin. It cross-reacts with leu-enkephalin.