Sera from Hartley guinea pigs vaccinated with a veterinary live spore anthrax vaccine were compared with sera from guinea pigs vaccinated with the human anthrax vaccine, which consists of aluminum hydroxide-adsorbed culture proteins of Bacillus anthracis V770-NP-1R. Sera from animals vaccinated with the spore vaccine recognized two major B. anthracis vegetative cell-associated proteins that were either not recognized or poorly recognized by sera from animals that received the human vaccine. These proteins, termed extractable antigens 1 (EA1) and 2 (EA2), have molecular masses of 91 and 62 kilodaltons, respectively. The EA1 protein appeared to be coded by chromosomal DNA, whereas the EA2 protein was only detected in strains that possessed the pXO1 toxin plasmid. Both of the extractable antigen proteins were serologically distinct from the components of anthrax edema toxin and lethal toxin. Following vaccination with the live spore vaccine, the EA1 protein was the predominant antigen recognized, as determined by electrophoretic immunotransblots. Vaccine trials with partially purified EA1 demonstrated that it neither elicits protective antibody against anthrax nor delays time to death in guinea pigs challenged intramuscularly with virulent Ames strain spores. In addition, animals vaccinated with sterile gamma-irradiated cell walls had significant antibody titers to the N-acetylglucosamine-galactose polysaccharide of B. anthracis but were neither protected nor had a delay in time to death following challenge.