There has been a regain of interest in the immunological applications of peptides assembled partly or totally from D-amino acids. Such peptides are much more stable to proteolysis than natural L-peptides and they have considerable potential as synthetic vaccines and as immunomodulators in T-cell responses. Retro-inverso, also called retro-all-D or retroenantio, peptide analogues that closely mimic the structure of protein antigens are obtained by assembling amino acid residues in the reverse order from that in the parent peptides and replacing L- by D-amino acids. Retro-all-D peptides corresponding to an immunodominant epitope of foot-and-mouth disease virus have been shown to elicit high levels of neutralizing antibodies in experimental animals. Certain retro-all-D peptide analogues of T-cell epitopes are able to bind to MHC class II molecules and may either lead to T-cell activation or inhibit deleterious T-cell responses.