The seroconversion rates and geometric mean concentrations (GMC) of IgG anti-PA for stored sera from U.S. military personnel immunized 3, 4, and 6 times with the U.S. licensed anthrax vaccine adsorbed were studied. Anti-PA IgG concentrations were measured by ELISA. All 246 vaccinees had low but detectable pre-immunization anti-PA IgG (GMC 1.83 microg/mL). Three doses elicited a GMC of 59.92 microg/mL and a seroconversion rate of 85.3%, four doses elicited a GMC of 157.44 microg/mL and 67.9% and the sixth of 276.95 microg/mL and 45.5%, respectively. The forth dose elicited 100% seroconversion compared to the pre-immunization level. These results should facilitate comparison between different immunization schedules and new vaccines.