Bacillus anthracis Tsiankovskii-I. This strain is unique as it contains both of the known virulence plasmids of B. anthracis and was successfully used to immunize livestock in the Soviet Union until 1960. The molecular basis for its attenuation and success as vaccine is not understood. Determination of the basis for attenuation will add to our knowledge of the pathogenesis of anthrax and may identify potential targets for new vaccines and therapies.