We have previously reported that α4β1 (but not α5β1) integrin- mediated melanoma cell adhesion is inhibited by removal of cell surface chondroitin sulfate glycosaminoglycan (CSGAG), suggesting that melanoma chondroitin sulfate proteoglycan plays a role in modulating the adhesive function of α4β1 integrin. In the current study, we demonstrated that α4β1 integrin binds to CSGAG. We have identified a peptide from within α4 integrin termed SG1 (KKEKDIMKKTI) that binds to cell surface melanoma chondroitin sulfate proteoglycan, indicating that SG1 represents a CSGAG binding site within the α4 integrin subunit. Soluble SG1 inhibits α4β1 integrin-mediated human melanoma cell adhesion to CS1. Polyclonal antibody generated against the peptide inhibits melanoma cell adhesion to CS1, and the inhibition is reversed by Mn2+ and an activating monoclonal antibody anti- β1 (8A2). Additionally, pretreatment of cells with anti-SG1 IgG inhibits the expression of the monoclonal antibody 15/7 epitope in the presence of soluble CS1 peptide, suggesting that anti-SG1 IgG prevents ligand binding by α4β1 integrin. These results demonstrate that α4β1 integrin interacts directly with CSGAG through SG1 site, and that this site can affect the ligand binding properties of the integrin.