SV40 transformants of human embryo fibroblasts showed an enhanced chemotactic response to fibronectin and conditioned medium of fibroblasts in comparison to the non-transformed cells. An even higher chemotactic response was characteristic for a malignant human fibrosarcoma cell line, whereas cells derived from low-grade malignant dermatofibrosarcoma showed a normal response in the Boyden chamber assay. The high stimulation of chemotaxis is paralleled by a reduced synthesis and an altered deposition of fibronectin in the pericellular matrix of these cells. Both high chemotactic response and lack of deposition of fibronectin may play a role in tissue invasion and formation of metastasis by malignant tumors.