Abstract: A process for the preparation of immunogens or diagnostic reagents that mimic an antigen or a pathogenic organism specific to a disease, essentially characterized by the following operations: identification of at least one antibody that reacts with the antigen or pathogenic organism specific to the disease; construction of phage libraries which display on the surface of the capsid oligopeptides, expressed from random sequence oligonucleotidic inserts introduced into a gene coding for a phage capsid protein using genetic manipulation techniques (for example using a plasmid engineered for the purposes of the invention, the genetic map of which is shown in the figure); selection of the phages that display on the surfaces of the capsid antigenic oligopeptides recognized by said antibody; optional use of the selected phages and/or fragments thereof and/or their derivatives for the formulation of diagnostic kits for the specific pathogenic agent, or in general for the diseases, including immunological disorders typ