The antimetastatic activities of synthetic peptides corresponding to fragments of the adhesion-related molecules, such as fibronectin and laminin, were examined. We prepared three peptides derived from the type III connecting segment domain (IIICS) of fibronectin: Glu-Ile-Leu-Asp-Val (EILDV), Glu-Ile-Leu-Asp-Val-Pro-Ser-Thr (EILDVPST), Arg-Glu-Asp-Val (REDV), and a laminin-related peptide, Tyr-Ile-Gly-Ser-Arg (YIGSR). Each peptide inhibited the experimental tumor metastasis of B16-BL6 melanoma, while EILDV had the strongest effect. The peptides conjugated with poly(ethylene glycol) (PEG) were more effective than the unmodified peptides in molar ratio terms. A mixture composed of PEG hybrids with EILDV, REDV and YIGSR significantly inhibited tumor metastasis.