We have investigated the anti-metastatic and anti-invasive activities of polypeptide analogues based on the Arg-Gly-Asp (RGD) adhesive signal in fibronectin, poly(RGD), poly(RGDS)[Arg-Gly-Asp-Ser] and poly(RGDT)[Arg-Gly-Asp-Thr]. These polypeptides containing repetitive RGD sequences were able to inhibit experimental and spontaneous lung metastases of B16-BL6 cells more effectively than the corresponding monomer peptides. In the spontaneous metastasis model, multiple i.v. administrations of these polymeric peptides before or after surgical excision of the primary tumor resulted in a significant reduction of lung tumor colonies. However, there was no significant difference in ability to inhibit spontaneous lung metastasis among poly(RGD), poly(RGDS) and poly(RGDT), although the carboxy-terminal amino acid residue (i.e., Xaa in -RGDXaa-) has been shown to play an important role in the expression of cell adhesive character. The treatment with poly(RGD) substantially prolonged the survival time for mice injected s.c. with B16-BL6 melanoma as compared with the untreated control. We also found that the polypeptides were potently able to inhibit the invasion and migration of tumor cells in vitro. Since these polypeptide analogues showed no antigenicity in the host and had no toxic effect on tumor cells in vitro, they may be potentially useful in the prevention of cancer metastasis.