Immunoblotting and immunochemistry were used to study the expression of a c-src gene-encoded protein in human lung tumors. The authors were the first to identify this otherwise rarely expressed protooncogene in as many as 60% of lung malignancies of various histogenesis. The expression of c-src protein was increased not only in neuroendocrine tumors (small-cell cancer and atypical carcinoid) but also in non-small-cell tumors such as adenocarcinoma, bronchoalveolar and squamous-cell lung cancer. A weak correlation between protein expression and degree of cell differentiation was established for squamous-cell tumors. The study failed to identify the oncoprotein in the normal tissue and benign lesions. Immunoblotting using Mab 327 monoclonal antibodies and the immunohistochemical method employing TBR polyclonal serum yielded similar data. To summarize, an increased expression of pp60c-src was observed in lung cancer of various histology.