The immunogenicity and reactogenicity of different doses of hepatitis A vaccine was studied in healthy adult volunteers. Vaccinees (105) were immunized with 6.25, 12.5 or 25 ng of HAV antigen, each dose administered at 0, 1 and 6 months (groups B, C and D); one group (group A) obtained three 6.25 ng doses at 0, 1 and 2 months. After one single dose high seroconversion rates ranging between 63 and 85% were observed in all four groups. All participants had seroconverted after the third dose, irrespective of the antigen content per dose and the vaccination schedule. Geometric mean titers after three doses were 439 IU/l (group A, month 3) and 1492, 963 and 2772 IU/l in groups B, C and D at month 7. One year after the first injection all vaccinees tested still showed antibody levels well above 10 IU/l. The vaccine was very well tolerated. Minor localized symptoms were observed mainly such as slight pain at the injection site. These symptoms were not dose related; no serious side effects occurred.