Uncoupled synthetic peptide representing the sequence of amino acids 141-160 of foot-and-mouth disease virus (FMDV) protein VP1 induced a virus-neutralizing antibody response in guinea-pigs. This response required incomplete Freund's adjuvant (IFA) for the primary inoculation and was dependent on the presence of an added cysteine residue with an unblocked sulphydryl group at the carboxy-terminus. Secondary immunization could be carried out in the absence of adjuvant. A study of the relative activities of nested sets of uncoupled peptides from 150-160 to 135-160 and 141-160 to 141-155 indicated that amino acids 146-156 were critical for the induction of virus-neutralizing antibodies and that extension to 137-160 further improved this response. Results of in vitro proliferation studies demonstrated that the carboxy-terminal residues on this peptide may form a T-cell epitope. The significance of these observations in the broader context of synthetic peptide vaccines is discussed.