We have previously shown that intraperitoneal immunization of BALB/c mice with the 14 amino-acid long synthetic peptides G/174-187 and BG/174-187, representing the region 174-187 of the G-glycoprotein from human (H) and bovine (B) respiratory syncytial virus (RSV), respectively, completely protects animals from infection with the corresponding virus. A current goal in vaccine development being the delivery of noninvasive protective antigens via mucosal surfaces, we have evaluated the immunogenicity and protective efficacy of the two peptides when administered to mice by the intranasal (i.n.) route in the presence or absence of the cholera toxin (CT) as a mucosal adjuvant. The two peptides given alone induced the production of RSV-specific circulating IgG, as revealed by ELISA titers of immune sera. When the peptides were administered intranasally with CT, the higher IgG antibody titer which was induced was within the same order of magnitude as that obtained following i.n. immunization with live RSV or intraperitoneal injection with the peptides, thus demonstrating the stimulatory effect of the CT adjuvant. Moreover, although the peptides fail to induce a detectable level of secretory IgA, all animals immunized i.n. with peptide BG/174-187 (plus or minus CT) and all those immunized with peptide G/174-187 mixed with CT were completely resistant to infection by the corresponding virus. To our knowledge, this is the first study reporting that complete protection against a natural pathogen can be elicited by mucosally delivered synthetic peptides. This supports the usefulness of synthetic peptides in prophylactic vaccination.