This is an open-label study to evaluate the immunogenicity and safety of raxibacumab in healthy adult male and female subjects. Subjects who have received raxibacumab >= 4 months ago will be enrolled and dosed as follows: A maximum of 25 subjects (to include 3 evaluable female subjects) will receive a second dose of raxibacumab equal to that of the previous dose >= 4 months following the first dose. Subjects will remain in house from Day 0 until Day 1 and will be followed for 70 days after receiving the second dose of raxibacumab. Raxibacumab has been shown to provide improved survival in rabbit and monkey anthrax spore challenge studies. Preliminary data from our rabbit pivotal efficacy study showed significant survival benefit for raxibacumab over placebo. Exposure to anthrax and resulting clinical disease can occur more than once, especially in individuals who do not develop protective immunity. Hence, if clinically indicated for the treatment of anthrax, there may be a requirement for the repeat administration of raxibacumab. The rationale of the study is to evaluate the immunogenicity and safety of repeat administration of raxibacumab with a >= 4 month interval between dosing.