Little research has been conducted on low booster doses in adults and only intradermal (i.d.) inoculation has been used with high rates of adverse reactions among subjects i.d. revaccinated. The present study compare a low intramuscular dose of recombinant hepatitis B vaccine with standard 20 micrograms revaccination. We studied 122 hospital workers 5 years after vaccination with recombinant hepatitis B vaccine who were randomly allocated to either the study group or control group. The study group received revaccination with 0.1 ml (2 micrograms) of recombinant hepatitis B vaccine and the control group 1.0 ml (20 micrograms). Two micrograms of i.m. vaccine was as effective as 20 micrograms i.m. in inducing antibody response and the antibody decrease after revaccination between groups was similar (P > 0.05) and independent of the initial concentration of anti-HBs after revaccination and dose (20 micrograms vs 2 micrograms). In conclusion, in subjects who had received a standard hepatitis B vaccination and had showed anti-HBs titres higher or equal than 10 mIU ml-1 after vaccination, low i.m. booster doses of vaccine give a rapid anamestic response similar to standard revaccination with less adversal reactions than low i.d. administered dose and would be an effective and less expensive alternative to revaccination against HBV of populations at high risk.