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Yersinia pestis produces a glycoprotein capsule, the biosynthesis of which appears to be temperature dependent. The fraction I (F1) component of this capsule is specific to Y. pestis and the detection of F1 antibodies is the basis for several serological tests. We report the cloning of the F1 gene and its expression in Escherichia coli using the phagemid vector λZAPII and a F1-specific monoclonal antibody. The recombinant F1 antigen had a molecular weight of 17 kDa, which proved to be identical to that of the F1 antigen produced by Y. pestis. The recombinant cells produced F1 antigen at 37°C but only minimal amounts at 27°C, suggesting that the genetic features affected by temperature in Y. pestis may be operating in the E. coli clone. It is not known if their similar molecular weights reflect the glycosylated nature of both proteins. F1 antigen purified from the E. coli recombinant induced a protective immune response in BALB/c mice challenged with up to 105 virulent Y. pestis. The resistance of immunized mice to plague infection correlated with high titers of F1 antibody. The cloned gene expresses an immunogenically competent F1 antigen suitable for use in plague serodiagnostics and vaccine development.