Cohort I (HIV-negative volunteers): At study entry a medical history and physical exam is performed, and volunteers complete a questionnaire. Blood samples are drawn weekly until Week 12, then at Weeks 14, 16, 20, and 24. Volunteers are followed for 24 weeks. Volunteers are offered the opportunity to participate in a lymphoid tissue substudy, which involves one to four sequential gut-associated lymphoid biopsies. Compensation for travel and for the inconvenience of study participation is provided.