A polypeptide useful as an immunogen element and characterized in that it is carried on the peptide sequence between amino acid residues 130-230 of the G protein sequence of the human respiratory syncytial virus of sub-groups A and B, or of the bovine respiratory syncytial virus, or on a sequence at least 80% homologous thereto. An immunogenic agent or pharmaceutical composition containing said polypeptide, and methods for preparing and using the same, are also disclosed.