1. Department of Virology, Research Institute for Microbial Diseases (Biken), Osaka University, Suita, Osaka 565-0871,1 and
2. Section of Serology, Institute of Immunological Science, Hokkaido University, Kita-ku, Sapporo 060-0815,2 Japan, and
3. Department of Microbiology, Immunology and Parasitology, Louisiana State University Medical Center, New Orleans, Louisiana 70112-13933