Inclusion Criteria:

Patients may be eligible for this study if they:

• Are between 18 and 32 years of age.
• Give written informed consent.
• Are available for follow-up for 2.5 months.
• Have an acceptable medical history by screening evaluation and brief clinical assessment.
• Are HIV-negative.
• Agree to use acceptable contraception and to not get pregnant during the study.

Exclusion Criteria:

Patients may not be eligible for this study if they:

• Have a suppressed immune system.
• Have participated in an HIV vaccine trial.
• Have liver disease, diabetes, or kidney problems.
• Have malignant skin cancer.
• Have autoimmune disease.
• Have used medication that suppresses the immune system.
• Have had psychiatric or medical problems or have job responsibilites that will keep them from participating in the study.
• Use illegal intravenous drugs.
• Have received any live vaccines within 60 days of study start.
• Have used experimental therapies within 30 days before study start.
• Have been vaccinated with certain experimental vaccines.
• Have a vaccination scar but have no record of having a vaccine.
• Have received a smallpox vaccine.
• Have taken blood products or immunoglobulin in the past 6 months.
• Have a fever on the day of vaccination.
• Have certain skin disorders, including eczema, exfoliative skin, lacerations that require stitches or burns larger than 2 x 2 cm.
• Live with, have sexual contact with, or work with anyone who is pregnant, less than a year old, has eczema, has certain skin disorders, has a disease or takes medication that suppresses the immune system.
• Have a condition that, in the opinion of the investigator, might interfere with the study.
• Are allergic to any parts of the vaccine.
• Are allergic to thimerosal, immunoglobulins, cidofovir, or probenecid.
• Are pregnant or breast feeding.