Adhesion to specific extracellular matrix molecules appears to be an important prerequisite for successful target organ colonization by metastasizing tumour cells. Interference in the adhesive function of malignant cells with antiadhesive agents is therefore one potential approach for preventing metastasis. Recently, synthetic peptides taken from the cell interaction sites of fibronectin have been characterized as inhibitors of cellular adhesion in vitro. Using these antiadhesive probes we have examined the role of cell adhesion to fibronectin in tumour metastasis using the B16-F10 murine melanoma model system. Two sequences from the IIICS cell-binding domain, the 25-mer CS1 peptide and the tetrapeptide Arg-Glu-Asp-Val (REDV), had no detectable activity, but the pentapeptide Gly-Arg-Gly-Asp-Ser (GRGDS), an active sequence from the central cell-binding domain, exhibited potent, dose-dependent inhibition, indicating a role for this cell recognition determinant in tumour metastasis. Under appropriate conditions GRGDS treatment afforded remarkable protection to the host; mice injected with melanoma cells and peptide were still alive 15 months after injection whereas mice injected with melanoma cells alone died within six weeks. Kinetic analyses of the retention of tumour cells in the lungs and of the vascular clearance rate of labelled GRGDS predict an early time frame of activity for the peptide. From the results of a variety of in vitro invasion and migration assays it appears that GRGDS may interfere with multiple, fibronectin-mediated adhesive and migratory events at different points of the metastatic cascade. In preliminary studies designed to optimize the therapeutic usefulness of GRGDS-like agents, peptide conjugates have been found to possess enhanced antiadhesive activity as well as an extended vascular clearance rate. In the future, therefore, these or related peptide derivatives may be potentially useful agents for the prevention of tumour metastasis.