Methods are disclosed for immunizing a mammal against B. anthracis using a composition of pure recombinant Protective Antigen (rPA), optionally in combination with truncated Lethal Factor polypeptide (LFn). Formulations of the pure rPA immunogen have little or no reactogenicity and therefore may be administered to a mammalian subject in very high doses of 50 &mgr;g to 1000 &mgr;g or more rPA, which is at least four times the amount of PA included per dose in conventional anthrax vaccines. Preferred immunogenic compositions are free of adjuvant and other undesired components, further enhancing the effectiveness and safety of the compositions. Methods for preparing the immunogenic compositions and for purifying rPA and LFn polypeptides also are disclosed.
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