IT has widely been assumed that the antigen thought to constitute the capsule or 'envelope' which surrounds plague organisms in vivo1 and which forms under suitable conditions in vitro2 is essential for full virulence of P. pestis in mice. This antigen has been isolated by Baker et al.3 and given the name Fraction 1 (F1). From their studies on the relative abilities of strains to produce F1 and toxin, Englesberg et al.4 correlated high virulence for mice with ability to produce maximal amounts of F1 and toxin.