The availability of synthetic antigens permitted a systematic elucidation of the molecular basis of antigenicity, as well as of other phenomena, and to establish the genetic control of immune response and its link to the major histocompatibility region of the species. Moreover, it allowed a conceptual approach to production of vaccines made up of synthetic fragments of relevant virus antigens. This was first demonstrated with a bacteriophage, MS-2, that can be efficiently inactivated with antibodies obtained with an immunogen in which a synthetic peptide corresponding to a sequence of 20 amino acid residues within its coat protein was covalently linked to a polymeric synthetic carrier. When the synthetic adjuvant N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) was chemically linked to the same molecule, the resulting immunogen was effective in aqueous solution. This approach has now been used for diphtheria: a synthetic peptide from the A-chain of the toxin, covalently attached together with MDP to the synthetic carrier multichain poly(D1-alanine) induced antibodies crossreactive with the diphtheria toxin as well as protection against the dermonecrotic activity of the toxin. A similar approach has been described also by R. Arnon and her colleagues in our laboratory for influenza virus, making use of a peptide with sequence 91-108 of the hemagglutinin of type A H3Nz strains attached to tetanus toxoid. The results indicated that the conjugate led to partial protection against A/Texas mouse-adapted influenza virus.