Normal human fibroblasts contain a cell type-specific glycoprotein antigen (SF) that is known to be slowly shed into the medium and to be present also in human serum. Immunofluorescence with anti-SF antibodies showed that SF antigen has a highly nonrandom fibrillar distribution in surface of normal fibroblasts. Simian virus 40-transformed fibroblasts also produced the SF antigen, as shown by radioimmunoassay or immunodiffusion tests, but it was not retained by the surface of these cells. This creates a major difference between the surfaces of normal and malignant cells.