Yersinia pestis fraction 1 capsular antigen (F1) is a plasmid (pFra)-encoded, proteinaceous capsule, synthesized in large quantities by the pathogen, and reported to confer antiphagocytic properties on Y. pestis by interfering with complementmediated opsonization. The protein is highly immunogenic and has been indirectly associated with eliciting a protective immune response in humans, as evidenced by the detection of high levels of anti-F1 antibody in F1-immunized volunteers [Ref84:Andrews et al., 1996].