 This study is conducted among a total of 1564 healthy adult men and women 18 to 61 years of age at five sites in the United States. Participants were randomized into one of 6 study groups with 260 participants per group. One group receives BioThrax given as currently licensed SQ with 6 doses followed by annual boosters; another group is given placebo IM 130 participants or SQ 130 participants in the currently licensed dosing regimen. The four other groups receive BioThrax IM in modified dosing regimens; placebo is given when a dose of BioThrax is omitted from the licensed dosing regimen. There are a total of 25 required visits for this study, during which all participants receive an injection of vaccine or placebo 8 injections total, have a blood sample drawn 16 or 17 total, and have an in-clinic examination for adverse events 22 total. 