A polysaccharide yielding an osazone resembling that of arabinose with a melting point of 166[degree]-168[degree]C was isolated from the supernatant of the Haffkine plague vaccine as well as from the specific soluble protein and, to a less extent, from the bacterial debris of both virulent and avirulent protective plague strains. It is absent in non-protective avirulent plague and pseudotuberculosis organisms. The protective substance of the plague bacillus is probably a saccharide-protein complex.