A total of 162 anti-HAV-negative healthy adults were immunized with a single high dose (1440 ELISA units = 1 ml) of inactivated hepatitis A vaccine and a booster was given at month 6. Antibodies were measured after modification of a commercial ELISA kit, enabling quantification of titres down to 6 mIU ml-1. After 14 days, 96% of vaccinees had positive anti-HAV titres (> 20 mIU ml-1), and after 30 days all vaccinees had seroconverted. The geometric mean titres were 150 mIU ml-1 and 380 mIU ml-1 respectively. The booster dose at month 6 elicited a dramatic (25-fold) increase in antibody titres from 125 to 3090 mIU ml-1. Thus a single dose may be a convenient alternative to immunoglobulin for inducing rapid seroprotection, with a booster dose at month 6 ensuring long-lasting protection.