T-cell epitopes within viral polypeptide VP4 of the capsid protein of foot-and-mouth disease virus were analyzed using 15-mer peptides and peripheral blood mononuclear cells (PBMC) from vaccinated outbred pigs. An immunodominant region between VP4 residues 16 and 35 was identified, with peptide residues 20 to 34 (VP4-0) and 21 to 35 (VP4-5) particularly immunostimulatory for PBMC from all of the vaccinated pigs. CD25 upregulation on peptide-stimulated CD4(+) CD8(+) cells-dominated by Th memory cells in the pig-and inhibition using anti-major histocompatibility complex class II monoclonal antibodies indicated recognition by Th lymphocytes. VP4-0 immunogenicity was retained in a tandem peptide with the VP1 residue 137 to 156 sequential B-cell epitope. This B-cell site also retained immunogenicity, but evidence is presented that specific antibody induction in vitro required both this and the T-cell site. Heterotypic recognition of the residue 20 to 35 region was also noted. Consequently, the VP4 residue 20 to 35 region is a promiscuous, immunodominant and heterotypic T-cell antigenic site for pigs that is capable of providing help for a B-cell epitope when in tandem, thus extending the possible immunogenic repertoire of peptide vaccines.