Synthetic vaccines for viral diseases can use defined regions of viral proteins as immunogens: the peptide sequence of amino acids 141-160 of the VP1 protein of foot and mouth disease virus (FMDV) elicits virus-neutralizing antibodies to protect guinea pigs, cattle and pigs either when coupled to a carrier protein or when administered in liposomes or in incomplete Freund's adjuvant. The immune response to these peptides is much lower than that to complete virus particles and the same sequence fused to the N terminus of beta-galactosidase did not produce a more potent immunogen than synthetic peptide alone. We report here an expression system for immunogenic epitopes linked to a carrier protein, hepatitis B core antigen, to form part of a virus-like complex which can present these epitopes to the immune system at high density. The immunogenicity of these structures approaches that of FMDV particles.