The study has examined how the plague bacillus expresses genes of two high-molecular weight (190 and 240 kD) iron-regulated membrane proteins (HMWP). The latter were purified and assayed for their immunobiological properties in vitro and in vivo. DNA-probing (at E.Carniel's Laboratory, Pasteur Institute, Paris) has indicated that Yersinia pestis strains having genes that code for HMWP synthesis circulate in the natural foci of plague which are situated in the former USSR. The use of HMWP in combination with live plague vaccine EV was found to greatly enhance its immunogenicity.