Fibronectin in human solid tumors was studied by indirect immunofluorescence staining of biopsy material. Altogether 73 tumors were examined, comprising 12 sarcomas, 3 melanomas 1 reticulum cell sarcoma, 39 carcinomas, 6 benign soft-tissue tumors and 12 benign epithelial tumors. In all sarcomas the individual tumor cells were surrounded by a network of fibronectin which was continuous with the stroma. The distribution of fibronectin was similar in the benign soft-tissue tumors. In contrast, no fibronectin was detected in the individual carcinoma cells or in their periphery. However, the reactive connective tissue stroma of carcinomas was strongly connective tissue stroma of carcinomas was strongly positive for fibronectin. This was true also for the stroma of benign epithelial tumors. These results show that, contrary to the situation in cell culture, in vivo sarcoma cells and benign soft-tissue tumor cells contain fibronectin in their pericellular matrix. On the other hand, fibronectin can be used to distinguish carcinomas from sarcomas in vivo.