Health care workers are the group at high hepatitis B virus (HBV) infection risk. Groups of physicians and nurses working in hospitals of Lublin have been vaccinated since 1989. The aim of this work was to assess the level of anti-HBs in different years after the basic course of vaccination. We have examined 166 persons, aged from 18 to 65 years, vaccinated with a yeast recombinant hepatitis B vaccine Engerix B (Smith Kline Biologicals). They completed the full cycle of vaccination according to the pattern of 0, 1, 6 months. Within the group of people who had got the third dose one year ago we found 100% of women and 88% of men with a protective level of anti-HBs (> or = 10 IU/l). Four years after the final inoculation differences between female and male population were even more significant: 93% of man had a protective level of anti-HBs. We have also found that humoral response was higher in the group of younger vaccinees (18-40 years). We propose that timing of booster vaccination should be scheduled on the basis of anti-HBs level. It seems to be necessary to control the level of anti-HBs at least 3 years after the last dose of vaccination.