A sub-immunizing dose of a synthetic peptide corresponding to the amino acids 141 to 160 region of protein VP1 from foot-and-mouth disease virus (FMDV), serotype O1, coupled to keyhole limpet haemocyanin (141-160KLH) has been shown to prime the immune system of guinea-pigs for an FMDV serotype-specific neutralizing antibody response to a second sub-immunizing dose of the same peptide. Optimal priming required an interval of 42 days between the priming dose and the booster dose. No priming was observed in the absence of adjuvant. The secondary response was not restricted by the carrier since animals primed with 141-160KLH could be boosted with uncoupled 141-160 or 141-160 coupled to tetanus toxoid. It has also been shown that uncoupled peptide 141-160 will prime for a neutralizing antibody response when it is incorporated into a relatively non-immunogenic carrier such as small unilamellar liposomes. These results indicate that the 141-160 peptide of FMDV, as well as containing an important neutralizing antibody site, can initiate its own T-helper cell response.