Sixty-four lung tumors were evaluated for the presence of immunoreactive neuron-specific enolase (NSE), bombesin (Bn), and chromogranin (Cg) to assess their value as markers for neuroendocrine cells in the histologic diagnosis of pulmonary neoplasms. Staining was correlated with the presence and density of neurosecretory granules (number of neurosecretory granules per unit cytoplasmic cross-sectional area) as determined by planimetry on electron micrographs. The cytoplasmic density of neurosecretory granules was significantly greater in the carcinoid tumors than in the small cell carcinomas (P less than 0.001). Neuron-specific enolase was localized in all of the neuroendocrine granule-bearing tumors but was also present in 57 per cent of the nonneuroendocrine carcinomas. Bombesin was present in 68 per cent of the neuroendocrine tumors and in less than 1 per cent of the nonneuroendocrine tumors. Staining for Cg appeared to correlate with the density of neuroendocrine granules, with staining in carcinoid tumors but no staining in small cell anaplastic carcinomas. A panel of antibodies may be required for the reliable identification of neuroendocrine lung tumors by immunohistochemical techniques.