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Background: The fatal disease caused by Bacillus anthracis is preventable with a prophylactic vaccine. The currently available anthrax vaccine requires a lengthy immunization schedule, and simpler and more immunogenic options for protection against anthrax are a priority for development. In this report we describe a phase I clinical trial testing the safety and immunogenicity of an anthrax vaccine using recombinant Escherichia coli-derived, B. anthracis protective antigen (rPA). Methodology/Principal Findings: A total of 73 healthy adults ages 18-40 were enrolled and 67 received 2 injections separated by 4 weeks of either buffered saline placebo, or rPA formulated with or without 704 mg/ml AlhydrogelH adjuvant in increasing doses (5, 25, 50, 100 mg) of rPA. Participants were followed for one year and safety and immunologic data were assessed. Tenderness and warmth were the most common post-injection site reactions. No serious adverse events related to the vaccine were observed. The most robust humoral immune responses were observed in subjects receiving 50 mg of rPA formulated with AlhydrogelH with a geometric mean concentration of anti-rPA IgG antibodies of 283 mg/ml and a toxin neutralizing geometric 50% reciprocal geometric mean titer of 1061. The highest lymphoproliferative peak cellular response (median Lymphocyte Stimulation Index of 29) was observed in the group receiving 25 mg AlhydrogelH-formulated rPA. Conclusions/Significance: The vaccine was safe, well tolerated and stimulated a robust humoral and cellular response after two doses., Published in the Public Library of Science (PLoS ONE), v5 i11 e13849, 2010. Prepared in cooperation with Walter Reed Army Institute of Research, Rockville, MD, and DynPort Vaccine Co. LLC, Frederick, MD.