The mechanism by which Trypanosoma cruzi, the protozoan parasite that causes Chagas' disease, becomes attached to mammalian cells is not well understood. Fibronectin is thought to participate in the attachment, and in this study the region of fibronectin that interacts with the surface receptors of T. cruzi trypomastigotes was investigated by testing the binding of the amino acid sequence Arg-Gly-Asp-Ser, corresponding to the cell attachment site of fibronectin to T. cruzi trypomastigotes. Peptides with the sequence Arg-Gly-Asp-Ser, but not Arg-Phe-Asp-Ser, Arg-Phe-Asp-Ser-Ala-Ala-Arg-Phe-Asp, Ser-Lys-Pro, Glu-Ser-Gly, or Ala-Lys-Thr-Lys-Pro, bound to the parasite surface and inhibited cell invasion by the pathogen. Monoclonal antibodies to the cell attachment domain of fibronectin also inhibited cell infection by the parasite. The immunization of BALB/c mice with tetanus toxoid-conjugated peptide induced a significant protection against T. cruzi. The data support the notion that the sequence Arg-Gly-Asp-Ser of cell surface fibronectin acts as a recognition site for attachment of the parasites.