Abstract: A synthetic antigenically active polypeptide, which includes the amino acid sequence:H-Asp-Ala-Glu-Gln-Arg-Gly-Glu-Leu-Ala-Ile-Arg-Asp-Ala-Asn-Ala-Arg -Leu-Ser-Glu-Leu-OH; a process for the preparation of such polypeptide; an antigenic agent comprising such polypeptide convalently bonded to an immunogenically active carrier; a pharmaceutical composition comprising such polypeptide or antigenic agent together with a pharmaceutically acceptable carrier or diluent; and a method for determining the presence or absence in a biological sample of antibodies monospecific to such polypeptide, which comprises introducing into the sample such polypeptide or such agent or such composition and determining the presence or absence of agglutination.