A peptide corresponding to the major immunogenic site of the protein VP1 of foot-and-mouth disease virus (FMDV) will elicit a protective neutralizing antibody response in guinea-pigs, cattle and pigs. The response is much greater when the peptide is presented as a linear dimer or tetramer and pigs receiving as little as 40 micrograms peptide have been protected against challenge infection. An even greater response is obtained when the peptide is presented as part of the core protein of hepatitis B virus. Moreover, responsiveness to the peptide in non-responder mice can be stimulated by the simultaneous inoculation of an appropriate T-cell epitope linked to the FMDV peptide.