1. Blood Systems Research Institute, San Francisco, California 94118, and Department of Medicine, University of California, San Francisco, California
2. Transmissible Diseases Department, American Red Cross Holland Laboratory, Rockville, Maryland
3. HIV Immunology Branch, Centers for Disease Control and Prevention, Atlanta, Georgia
4. Division of HIV/AIDS Prevention, Centers for Disease Control and Prevention, Atlanta, Georgia
5. Blood Systems Research Institute, San Francisco, California 94118, and Department of Laboratory Medicine, University of California, San Francisco, California