Secretion of outer membrane vesicles (OMV) presents an important phenomenon in Gram-negative bacteria and they play multiple roles in their lifestyle including virulence and host-pathogen interaction. Francisella tularensis secretes unusually shaped tubular OMV filled with immunoreactive material and virulence factors. Mice were immunized with OMV isolated from Francisella tularensis subsp. holarctica, fully virulent strain FSC200. Their sera were then used for detection of immunoreactive proteins.