A Phase 1, randomized, double-blind, positive controlled, increasing dose clinical trial in healthy adult subjects at multiple sites. The study will assess safety and immunogenicity of two adenovirus vaccine candidates against anthrax compared to the positive control, Anthrax Vaccine Adsorbed (AVA). The trial will enroll 108 subjects in the anthrax vector vaccine arms and 12 subjects in the AVA positive control subjects. The study will look at three different dose of oral dosages of Ad4-PA (protective antigen) and Ad4-PA-GPI (10^9, 10^10, 10^11 vp/dose)as well as 3 vaccine administration schedules (1 and 15 days; 1 and 29 days; 1, 15, and 29 days); 2 of 3 schedules will include an intramuscular (IM) AVA booster immunization, 1 schedule will include 3 vaccine administrations of Ad4-PA or Ad4-PA-GPI (glycosylphosphatidylinositol) alone, and 1 schedule will include 3 vaccine administrations of AVA alone.