The Y. pestis CO92 strain used in the challenge studies was originally obtained from T. Quan, Centers for Disease Control and Prevention, Fort Collins, Co. It was isolated from the sputum of a human case of pneumonic plague [32] . The Y. pestis CO92 was grown and inoculum prepared for challenges essentially as described previously [33] . A Y. pestis pgmstrain, which was originally isolated from Y. pestis CO92, and used in the antibody binding studies described below was obtained from Susan L. Welkos (USAMRIID, Frederick, MD).