The safety and immunogenicity of an inactivated hepatitis A virus vaccine were assessed in 101 healthy adults. Seronegative persons with normal serum aminotransferase levels were grouped according to age: Group 1 (n = 24) and group 3 (n = 22) were between 18 and 40 years of age, and group 2 (n = 25) and group 4 (n = 30) were older than 40 years. Groups 1 and 2 received vaccine on a 0-, 1-, and 2-month schedule (schedule A), and groups 3 and 4 received the vaccine on a 0-, 1-, and 12-month schedule (schedule B). Of the 101 vaccinated subjects, 98 (97%) seroconverted with antibody titers to hepatitis A virus of > or = 20 IU per liter after the first dose, and all subjects seroconverted after the second dose. The geometric mean titers a month after the third vaccine dose were significantly greater (P < .03) on both schedules for younger subjects (schedule A, 1,743 IU per liter, and schedule B, 7,882 IU per liter) than for older subjects (schedule A, 826 IU per liter, and schedule B, 4,279 IU per liter). Also, the differences in geometric mean titers a month after the third dose were significantly greater (P < .001) for subjects in both age groups on schedule B (group 3, 7,882 IU per liter, and group 4, 4,279 IU per liter) than for those on schedule A (group 1, 1,743 IU per liter, and group 2, 826 IU per liter). The hepatitis A virus was well tolerated, with mild discomfort at the injection site being the main side effect. This vaccine is both safe and highly immunogenic.