1. Biologics Branch, Scientific Resources Program, Centers for Disease Control and Prevention, Atlanta, Georgia.
2. Present address: Bioterrorism Preparedness and Response Program, Centers for Disease Control and Prevention, Department of State Health Services, Austin, Texas.
3. Present address: Division of Bacterial and Mycotic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia.
4. Immunology Division, Department of Pathology, Cambridge University, Cambridge, United Kingdom.