Qualitative and quantitative alterations of human tenascin (TN) expression in virally transformed lung fibroblasts and in lung tumor tissues were investigated using S1 nuclease protection analysis in comparison with those of fibronectin (FN). Transformed fibroblasts and fetal lung tissues expressed more TN mRNA with an extra sequence encoding the sixth FN type III repeat than did normal cells and adult tissues. The splicing pattern of TN mRNA was also altered in many lung cancer tissues, showing increased or sometimes decreased expression of the TN mRNA with the extra sequence when compared with their surrounding normal tissues. These results provide additional evidence for the oncodevelopmental regulation of alternative RNA splicing in human lung tissues, first observed with FN mRNA (F. Oyama, et al., Cancer Res., 50: 1075-1078, 1990). Quantitative analysis of the levels of TN and FN mRNAs showed that the ratio of TN mRNA to FN mRNA was significantly increased in transformed fibroblasts and in some lung tumor tissues, when compared with their normal counterparts. Among different types of lung tumors, a significant increase of the TN/FN ratio was observed with most squamous cell carcinoma but with only a small fraction of adenocarcinoma. Since TN has been shown to inhibit cell adhesion to FN, the altered ratio of TN mRNA to FN mRNA may well affect the adhesive and migratory properties of tumor cells in lung cancer tissues.