The protective effects of polyclonal antisera produced by injecting guinea pigs with protective antigen (PA), the chemical anthrax vaccine AVA, or Sterne spore vaccine, as well as those of toxin-neutralizing monoclonal antibodies (MAbs) produced against PA, lethal factor, and edema factor, were examined in animals infected with Bacillus anthracis spores. Only the anti-PA polyclonal serum significantly protected the guinea pigs from death, with 67% of infected animals surviving. Although none of the MAbs was protective, one PA MAb caused a significant delay in time to death. Our findings demonstrate that antibodies produced against only PA can provide passive protection against anthrax infection in guinea pigs.