Seventy-one anti-hepatitis A virus (HAV) negative volunteers were immunized against hepatitis A. An inactivated hepatitis A vaccine (HAVRIX, SmithKline Beecham), derived from tissue cell cultures, at single doses of 720 ELISA units was used following a schedule of vaccinations at month 0.1 and 6. The vaccines were tested for the presence of HAV antibodies 1 month after each vaccination and then after 2, 3, 4 and 5 years. The annual decrease of anti-HAV titres was 25%. Five years after vaccination a protective antibody titre, varying between 20 and 5200 mIU ml-1, could be demonstrated in all 47 retested volunteers with a geometric mean titre (GMT) of 442 mIU ml-1. Levels of anti-HAV-antibodies following active immunization were significantly higher in female volunteers. This could be consistently demonstrated throughout the observation period. Based on these data the antibody persistence was calculated over time. GMTs at protective levels higher than 20 mIU ml-1 can be expected to persist for at least 15 years.