To study the feasibility of using inactivated hepatitis A vaccine for rapid immunization of US soldiers, 276 randomized seronegative volunteers received one of four regimens: two injections, on day 0 or one each on day 0 and 14, day 0 and 30, or day 0 and 180. A third dose was given on day 380. Among the 256 recipients of two doses, 99% responded with antibody (by ELISA) with few symptoms. A higher percentage of recipients of both doses on day 0 had antibody at day 14 (68% vs. 52% of all others, P < .03). The highest antibody concentrations (711 mIU/mL on day 240) were observed in subjects given a second dose on day 180. Recipients of the third injection developed a median 15-fold rise in antibody within 2 weeks. Virus-neutralizing antibody was detected in high titers after the third dose and neutralized strains of hepatitis A virus from several countries. Vaccines containing 1440 ELISA units of antigen may be useful for rapid immunization.