To assess the safety and immunogenicity of a booster dose of hepatitis B vaccine in children who have received a 3-dose primary series of either RECOMBIVAX HB or ENGERIX-B. The primary vaccination series (was given 4 to 8 years prior to study entry and consisted of a licensed hepatitis B vaccine product (either RECOMBIVAX HB or ENGERIX-B). The booster dose given in this study will be either an investigational Merck product (Modified Process Hepatitis B Vaccine) or licensed ENGERIX-B vaccine.