A synthetic peptide derived from the B1 chain of laminin, F-9, as well as two peptides derived from the 33-kilodalton fragment of fibronectin, I and II, directly promoted the adhesion of K-1735-M4 metastatic murine melanoma cells. In competition assays, adhesion of these cells to laminin was inhibited by excess soluble peptide F-9. Peptides F-9, I, and II specifically bound 3H-heparin, both by direct binding assays and indirect competition assays. 3H-heparin binding to peptide F-9 was specific as determined by competition with excess unlabeled heparin, dextran sulfate, and dermatan sulfate. These findings suggest that melanoma cell surface associated heparin-like molecules may act as receptors for these domains of laminin and fibronectin.