We have examined the influence of fibroblasts on the invasive and migratory potential of highly metastatic melanoma B16-BL6 and weakly metastatic B16-F1 cells in vitro. Co-culture of B16-BL6 cells with a fibroblast monolayer without cellular contact in a Transwell chamber more effectively induced tumor-cell invasion into Matrigel basement membrane than co-culture of B16-F1 cells with a fibroblast monolayer. The activity was closely correlated with the chemotactic migration of tumor cells toward the fibroblast monolayer. We also found that the conditioned medium (CM) from the co-culture of fibroblasts with B16-BL6 cells without cellular contact, i.e., CM (B16-BL6/fibroblast), rather than from co-culture with B16-F1 cells, could potentially promote the migration of tumor cells of both types. Tumor cells did not chemotactically migrate to the CM (B16-BL6), CM (B16-F1) or CM (fibroblast). Antibodies against TGF-beta 1 or FN almost completely abolished the chemotactic migration of B16-BL6 cells to the CM (B16-BL6/fibroblast) or CM (TGF-beta 1-treated fibroblast) when these antibodies were co-incubated with fibroblasts and either B16-BL6 or TGF-beta 1. In contrast, the anti-EGF antibody did not show any inhibitory effects. Analysis of amounts of TGF-beta 1 or FN in various CM using ELISA plates, and using their specific antibodies, revealed that the concentration of TGF-beta 1 in the CM (B16-BL6) was slightly higher than in the CM (B16-F1), and the amount of FN in the CM (B16-BL6/fibroblast) was twice as high as in the CM (B16-F1/fibroblast). These results suggest that TGF-beta 1 released from B16-BL6 cells can stimulate fibroblasts to produce FN; consequently, the tumor cells were able to chemotactically migrate toward the released FN, and the differences in invasive and migratory activities towards fibroblasts in B16-BL6 and B16-F1 cells may in part be due to the amounts of TGF-beta 1 from tumor cells and of FN from TGF-beta 1-stimulated fibroblasts.