To investigate the nature of various endocrine cells immunoreactive for human chorionic gonadotropin (hCG alpha) in the lung, immunoelectron microscopic study was performed on fibrotic adult lungs and endocrine neoplasms of the lung. The mode of localization of hCG alpha and the size profile of hCG alpha granules were different among endocrine cells under various proliferative conditions. The population of hCG alpha granules in the grouped type of endocrine cells was more variable with a shift to smaller size (mean area: 1.395 x 10(-2) microns 2, mean maximum diameter: 149.8 nm), than that in solitary ones (1.493 x 10(-2) microns 2, 155.4 nm). Tumorlet endocrine cells had larger hCG alpha granules (1.800 x 10(-2) microns 2, 171.3 nm) without change of SD of size parameters. In carcinoid tumors, the size profile of hCG alpha granules was considerably different from that in the three types described above. Moreover, hCG alpha granules were significantly smaller in size in carcinoid tumors without lymph node metastasis (2.295 x 10(-2) microns 2, 189.8 nm) than those in malignant carcinoid tumors with metastasis (3.368 x 10(-2) microns 2, 230.5 nm). The population of hCG alpha granules in atypical endocrine tumor was the parallel shift to a larger scale (6.251 x 10(-2) microns 2, 307.5 nm) from that of malignant carcinoids and the distribution pattern was different from that in benign carcinoids. In small cell carcinoma of the lung, hCG alpha immunoreaction was preferentially present in perinuclear space and dilated rough endoplasmic reticulum. The mode of localization of hCG alpha and the size profile of hCG alpha granules, representing specific features of intracellular processing of hCG alpha, may be closely related with some qualitative changes in the neoplastic process of pulmonary endocrine cells.