The invention concerns the identification and use of neogenin receptor-binding domains of members of the repulsive guidance molecule (RGM) protein family as well as polypeptide fragments derived therefrom. The inventive domains, i.e. peptide fragments are suited as agents for the active or passive immunization of individuals as well as diagnostic and therapeutic agents for use with diseases or pathological conditions in whose origin or progression, a member of the RGM family and a cellular receptor assigned to this molecule are involved. The invention also concerns monoclonal and polyclonal antibodies directed against the inventive binding domains and against the polypeptides derived therefrom, and to method for producing the inventive domains, polypeptides and antibodies.