1. Virology Cattle Research Unit, National Animal Disease Center, USDA, Agricultural Research Service, PO Box 70, Ames, IA 50010
2. National Veterinary Services Laboratories, USDA, Animal and Plant Health Inspection Service, Veterinary Services, PO Box 844, Ames, IA 50010
3. Laboratory of Persistent Viral Diseases, Rocky Mountain Laboratories, NIH, National Institute of Allergy and Infectious Diseases, Hamilton, MT 59840
4. Department of Virology, New York State Office of Mental Retardation and Developmental Disabilities, Institute for Basic Research in Developmental Disabilities, 1050 Forest Hill Road, Staten Island, NY 10314