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The distribution of tenascin immunoreactivity was analyzed in nonneoplastic lung tissue, benign lung tumors, and different types of lung carcinomas. In nonneoplastic lung tissue, tenascin could be observed in the basement membranes of the bronchial epithelium and endothelial cells, smooth muscle cells, and bronchial cartilage. Strong tenascin immunoreactivity was seen in the stroma of all the carcinomas of various histologic types. The staining intensity was stronger in the stroma of squamous cell carcinomas than in the stroma of the other types of lung carcinomas. In 10 of 27 squamous cell carcinomas, a granular intracytoplasmic reactivity could also be observed in a subpopulation of tumor cells. Similar intracytoplasmic reactivity was observed in 2 of 27 adenocarcinomas and in both adenosquamous carcinomas. In other types of lung tumors, individual cells did not have intracytoplasmic tenascin, except for one case of leiomyoma, which showed a weak, linear, intracytoplasmic tenascin reactivity. In lung hamartomas, tenascin could be seen in the cartilaginous component of the tumor and in the areas of basement membranes of the bronchial epithelium. In the carcinoid tumors, the stroma displayed a faint positivity for tenascin. These results show that tenascin is widely expressed in the stroma of lung carcinomas. A proportion of lung carcinomas also expressed intracytoplasmic tenascin immunoreactivity, suggesting that tumor cells may be able to synthesize tenascin. In the lung, tenascin positivity is not, however, restricted to malignant neoplasms, as evidenced by the presence of tenascin in nonneoplastic lung parenchyma and in some benign lung tumors.