The specific objective of this study is to evaluate the immune response to a five microgram dose of recombinant hepatitis B vaccine among 5-6 year old and 10-14 year old children who received the primary recombinant hepatitis B vaccine series beginning at birth. The concentration of antibodies to hepatitis B surface antigen (anti-HBs) will be measured immediately before administering the vaccine, and compared with levels in serum drawn 1, 2 and 4 weeks afterwards. A rapid antibody response (anamnestic response) indicates that immune memory, and therefore immunity to HBV infection, is preserved. The frequency and magnitude of the anamnestic response for the group of older children will be compared to that of the younger group.