A serological correlate of vaccine-induced immunity was identified in the rabbit model of inhalational anthrax. Animals were inoculated intramuscularly at 0 and 4 weeks with varying doses of Anthrax Vaccine Adsorbed (AVA) ranging from a human dose to a 1:256 dilution in phosphate-buffered saline (PBS). At 6 and 10 weeks, both the quantitative anti-protective antigen (PA) IgG ELISA and the toxin-neutralizing antibody (TNA) assays were used to measure antibody levels to PA. Rabbits were aerosol-challenged at 10 weeks with a lethal dose (84-133 LD(50)) of Bacillus anthracis spores. All the rabbits that received the undiluted and 1:4 dilution of vaccine survived, whereas those receiving the higher dilutions of vaccine (1:16, 1:64 and 1:256) had deaths in their groups. Results showed that antibody levels to PA at both 6 and 10 weeks were significant (P<0.0001) predictors of survival.