Passive protection by human or mouse anti-F1 or anti-V mAbs and Y. pestis challenge studies Antibodies to be evaluated for their efficacy against a plague challenge were given intraperitoneal (i.p.)(500 mg per mouse, except when stated differently in the figure legend) to 6-10 week old BALB/c mice 24 h before they were challenged or at time points post-challenge as indicated in the figure legend. The challenge dose was prepared from frozen stocks of Y. pestis CO92 that were streaked on tryptose blood agar slants and incubated at 28uC for 48 h. After the incubation period, the slants were rinsed with 10 mM potassium phosphate buffer, pH 7.0, and cell density adjusted to the required density with the same buffer. Mice were given the challenge dose of LD 50 ,25-40, subcutaneously in 0.2 ml, where 1 LD 50 is equal to 1.9 cfu [37] and observed for at least 21 days. Research was conducted in compliance with the Animal Welfare Act and other federal statues and regulations relating to animals and experiments involving animals and adheres to principles stated in the Guide for the Care and Use of Laboratory Animals, National Research Council, 1996. The facility where this research was conducted is fully accredited by the Association for Assessment and Accreditation of Laboratory Animal Care International. The studies involving mice were approved by the IACUC at the U.S. Army Medical Research Institute of Infectious Diseases, animal protocol number AP-07-040.