Small cell lung cancer is distinguished from other lung cancer histologic types by possessing a variety of neuroendocrine properties. Anti-Leu-7 is a monoclonal antibody that recognizes a 110,000-dalton molecular weight glycoprotein initially described on natural killer cells and subsequently reported on a variety of normal and malignant neural and neuroendocrine cell types. We have found intense anti-Leu-7 binding to a large number of small cell lung cancers, while other lung cancer types were negative or showed only weak and focal binding. Other antigens expressed by natural killer cells, lymphocytes, and monocytes were never or less often expressed on small cell lung cancer cells. In addition, we report for the first time anti-Leu-7 binding by carcinoids, carotid body tumors, pheochromocytomas, endocrine cells of the fetal bronchus and the adult intestine, and select pancreatic islet cells. Anti-Leu-7 binding by small cell lung cancer is consistent with a derivation from pulmonary precursor cells, and anti-Leu-7 staining is clinically useful for the identification of human neuroendocrine tumors of the amine precursor uptake and decarboxylation (“APUD”) type.