WE have previously reported that pathogenicity of Vibrio cholerae was suppressed by introducing P or V plasmids and that this effect was due to decreased toxin production1. This work has been extended to show that cells harbouring both P and V plasmids become attenuated and do not cause experimental cholera. Such attenuated strains are attractive candidates for live oral vaccines if it can be shown that the immunogenicity of plasmid-harbouring strains is not altered and that they remain stable. We provide here convincing evidence that the cells are not cured of their plasmids in vitro or in vivo and do not lose their immunogenic potential.