To investigate if cowpea mosaic virus (CPMV) particles can be used to express foreign protein sequences, oligonucleotides encoding an epitope derived from VP1 of foot-and-mouth disease virus (FMDV) were cloned into the region of the CPMV genome encoding the small (S) coat protein. The chimeras were designed so that the foreign sequence was expressed either as an insertion or as a replacement for part of the wild-type sequence. While RNA from both chimeras was able to replicate in cowpea protoplasts only the construct containing the FMDV sequence as an insertion was able to direct capsid formation and infect whole cowpea plants. The modified S protein produced in plants infected with the insertion derivative reacted with FMDV-specific antiserum. These results show that CPMV can be used as an antigen presentation system and raises the possibility of producing vaccines in plants using a RNA virus-based vector.