Exactly how the human anti-F1 252 m mAb is able to protect mice may be directly related to the presence of F1 antigen on the surface of the plague organism. The F1-antigen has been reported to be anti-phagocytic [22, 23] . The ability of macrophages to take up the plague organism is directly related to the lack of the F1antigen, and resistance to phagocytosis is related to the presence of the F1-antigen. The binding of the human anti-F1 252 m mAb to the surface of the plague bacilli or opsonization may trigger phagocytosis of encapsulated bacilli into macrophages, thereby allowing phagocytic cells to clear the host of the pathogen.