Previous observations have demonstrated that fibronectin is deposited in high abundance in basal cell carcinoma stroma. In this study, the nature of fibronectin and the site of its synthesis were explored in 10 basal cell carcinomas of the nodulo-ulcerative type by immunocytochemistry and in situ hybridization. First, simultaneous localization of epithelial tumor cell islands and fibronectin epitopes was carried out by double immunofluorescence staining with monoclonal anti-cytokeratin antibodies and polyclonal fibronectin antibodies, the latter recognizing both the cellular and plasma types of the protein. Large amounts of fibronectin were deposited in the basal cell carcinoma stroma, with the highest concentration present in the immediate proximity of the epithelial cell islands. Immunofluorescence with a monoclonal anti-fibronectin antibody, which is directed against the ED-domain of cellular fibronectin and does not recognize the plasma type of fibronectin, revealed essentially the same staining pattern as that obtained with the polyclonal anti-fibronectin antibody. This observation suggested that fibronectin in BCC was predominantly of the cellular type. Second, in situ hybridizations, utilizing a human fibronectin specific cDNA, demonstrated that the highest concentration of fibronectin mRNA was found in the most peripheral cell layer of the epithelial tumor islands. The presence of fibronectin mRNAs in the tumor cells of the central regions of the islands, as well as within occasional stromal cells, was also noted. Thus, two lines of evidence suggest that the epithelial tumor cells are predominantly responsible for the synthesis and deposition of fibronectin in basal cell carcinoma. The presence of fibronectin may explain the characteristic biologic behavior of basal cell carcinomas, including low degree of metastatic potential and local destructive nature of the tumors.