The safety and immunogenicity of an inactivated hepatitis A vaccine (HM175) were evaluated in 151 seronegative health professionals (age range, 21-65 years; mean, 30). A 720-ELISA unit dose was administered to 78 vaccinees at 0, 1, and 6 months and to 73 vaccinees at 0, 1, and 12 months. Seroconversion rates were 90% in both groups 1 month after the first inoculation and 99% and 100%, respectively, 1 month after the second inoculation. Geometric mean antibody titers (GMTs) 1 month after the third inoculation were highest in the group vaccinated at 0, 1, and 12 months. GMTs were higher in women than in men. The vaccine was well tolerated; the most frequent side effect was transient soreness at the site of inoculation. No serious adverse reactions were observed. Thus, HM175 inactivated hepatitis A vaccine is safe and highly immunogenic.