Loss of protective anti-HBs levels (less than 10 IU/l) was noted in 5 (13%) of 38 well documented healthy responders to hepatitis B vaccine 30 months after completing the initial standard vaccination series. Revaccination with a single booster injection of 20 micrograms hepatitis B vaccine intramuscularly resulted in anti-HBs levels well above those initially obtained, thus confirming considerable immunological memory. Both decline prior to and rise after booster injection were proportional to the anti-HBs level obtained initially. The antibody production after a single booster injection was closely monitored in 13 individuals. A swift response was observed from day 4 onwards in all subjects. Based on passive immunization data and in vitro infection of human hepatocytes, this time delay is likely to permit infection of hepatocytes. Therefore, until further data on longterm follow-up of vaccinated in individuals in whom anti-HBs levels have dropped to less than 10 IU/l reveal compelling evidence to the contrary, booster injections remain mandatory for those individuals at risk.