Healthy normal volunteers between 18 and 60 years of age may be eligible for this study. Candidates will provide a medical history, including information on sexual behaviors and drug use. They will have a physical examination and blood and urine tests. Women will also have a pregnancy test. Women enrolled in the study must either be infertile (e.g., due to menopause or hysterectomy) or must agree either to abstain from heterosexual sex or to practice birth control for at least 21 days before beginning the study and throughout its duration.