The expression of transforming growth factor-alpha (TGF alpha) was assessed by immunohistochemical staining in 52 human lung tumor samples. All of the 8 small cell lung cancers were negative whereas all of the 18 adenocarcinomas and 23 of the 26 squamous cell carcinomas showed positive immunoreaction to TGF alpha. Distribution of TGF alpha stainings in the squamous cell carcinomas was weaker and more heterogeneous as compared to the adenocarcinomas. Ultrastructural localization of TGF alpha in the squamous cell lung carcinomas by indirect immunogold staining revealed that TGF alpha is present in the cytoplasm as well as the cell membrane but not in the nucleus. This suggests that the lung cancer cells are not only the producer of TGF alpha, but also the target cells of the TGF alpha action. The expression of TGF alpha in lung tumors may be useful diagnostically in differentiating small cell lung cancer from non-small cell lung cancer and may also be important in the study of the biological properties of primary lung cancers.