1. From the Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD, Children's Hospital National Medical Center, Washington, DC, and the Division of Infectious Disease, Johns Hopkins Hospital, Baltimore, MD (address reprint requests to Dr. Kapikian at the National Institutes of Health, Bldg. 7, Room 103, Bethesda, MD 20014).