The capsule that surrounds Yersinia pestis cells is composed of a protein-polysacchride complex; the purified protein component is fraction I (F1) antigen. We report the cloning of the cafl gene and its expression in Escherichia coli using the vector pETl02/D-TOPO and the F1-specific monoclonal antibody. The recombinant F1 (rF1) antigen had a molecular size of 17.5 kDa, which was identical to that of the F1 antigen produced by Y. pestis. Recombinant F1 protein was found to react to polyclonal antiserum to Y. pestis Fl. Recombinant F1 was purified by ProBond purification system and induced a protective immune response in BALB/c mice challenged with up to 10$^5$ virulent Y. pestis. Purified rF1 protein was used in an ELISA to evaluate the ability of a method to detect antibodies to Y. pestis in animal sera. These results strongly indicated that the rF1 protein is a suitable species-specific immunodiagnostic antigen and vaccine candidate.