The long-term persistence of anti-HBsAg above 10 mUI is conventionally believed to protect against natural infection with hepatitis B virus, while it is not yet clear what is the clinical significance of the fall to below 10 mUI in antibody levels. In our opinion, an important method for evaluating the effectiveness of the vaccine lies in comparing the duration of vaccine-immunity with that following the disease, the later being held to provide life-long protection. In this view, we examined the sera samples of 69 subjects (56 healthy people, and 13 drug-addicts) previously affected from HBV infection looking for anti-HBsAg with an Elisa method. The subjects were aged between 4 months and 73 years and had all suffered from acute fully recovered icteric hepatitis in the previous 2 to 10 years. All the drug addicts (13 males) and 4 healthy children from the group had no or negligible levels of antibodies; 3 out of 8 old people (60 or more yrs old) showed no antibodies only 4-5 yrs after having been infected. In the remaining group of 44 healthy people 14 cases were observed which had antibody levels 10 mUI, regardless of age, sex or occupation; the greater the time between the infection and the test, the greater the probability of finding negligible antibody levels. We conclude that the reduction in antibody levels may not be indicative of a loss of immunity: many of our cases have shown low or undetectable antibody levels, a few years after full recovery from an acute attack of hepatitis B.