 This study is a 43-month prospective, randomized, double-blind, placebo-controlled comparison of immunogenicity and reactogenicity elicited by BioThrax given by different routes of administration SQ versus IM and dosing regimens as many as 8 doses versus as few as 4 doses. Sterile saline is used as the placebo where doses are dropped in regimens using AVA, and in the all-placebo study group. 