This study includes normal volunteers and patients who have had a blood transplant. Healthy volunteers between the ages of 18 and 60 who have previously been vaccinated with the hepatitis B or tetanus/diphtheria vaccine and have not had hepatitis B may be eligible for this study. Patients enrolled in a blood cell transplant study at NIH who are between 5 and 60 years old, have been vaccinated against tetanus/diphtheria, and have not had hepatitis B may be eligible. Candidates will be screened with a medical history and blood tests.