The aims of this study are to determine:

1. the degree of immunity against hepatitis B,
2. factors associated with immunity against hepatitis B,
3. if the amount of antigen provided in a booster dose (used to measure the degree of immune memory) affects the body's response to the booster, and
4. if the age at which the vaccination was initiated during infancy affects duration of protection against hepatitis B among adolescents 16-19 years of age born in the United States who were immunized against hepatitis B as infants. These data will be important to evaluate the potential need for a possible booster dose of hepatitis B vaccine among adolescents.

Study subjects eligible for enrollment will:

• have blood drawn for baseline serologies
• receive the hepatitis B booster dose (randomly determined to be either a 10 ug or 20 ug dose)
• have blood drawn again 14 days later to determine if the booster dose has engendered an anamnestic response (a response attributed to prior immunity rather than a new response to the vaccine)

The primary hypothesis is that the majority of young people will exhibit immune memory for hepatitis B in response to a booster dose of vaccine.

Inclusion Criteria:

• 16-19 years of age
• born in the United States
• received the full hepatitis B vaccination series by 12 months of age
• received the third dose of hepatitis B vaccine within 10 months of the first dose

Exclusion Criteria:

• a history of hepatitis B disease
• being born to a mother with hepatitis B at the time of the participant's birth
• having received hepatitis B immune globulin as an infant
• immunosuppression including HIV or chronic/current steroid use (use > 10 days duration within the 3 months of enrollment)
• renal insufficiency or dialysis
• pregnancy (due to theoretical risk of different immune response among pregnant females)
• ever receiving a booster (fourth) dose of hepatitis B vaccine
• history of allergy to the hepatitis B vaccine
• receipt of blood products or IgG within 3 months of enrollment
• having received another vaccine within 2 weeks of enrollment