Adhesive interactions between cells and the extracellular matrix occur at several stages of metastasis. Such interactions might be inhibited by synthetic peptide probes derived from the cell-binding regions of matrix molecules. Gly-Arg-Gly-Asp-Ser (GRGDS) is a pentapeptide sequence that appears to be critical for cell interaction with fibronectin. Coinjection of GRGDS with B16-F10 murine melanoma cells dramatically inhibited the formation of lung colonies in C57BL/6 mice. Two closely related control peptides, in which specific amino acids within the GRGDS sequence were transposed or substituted, displayed little or no activity. Inhibition by GRGDS was dose-dependent, noncytotoxic, and did not result from an impairment of cellular tumorigenicity. GRGDS may function by inhibiting tumor cell retention in the lung since radiolabeled B16-F10 tumor cells injected with the peptide were lost at a substantially greater rate than control cells.