1. Laboratory Animals Breeding Facilities, Osaka University Medical School, 2-2 Yamadaoka, Suita 565, Osaka, Japan; Department of Virology, Kurume University School of Medicine, Kurume 830, Japan
2. Laboratory Animal Center, Keio University School of Medicine, Shinjuku-ku, Tokyo 160, Osaka, Japan
3. Central Institute for Experimental Animals, 1430 Nogawa, Miyamae-ku, Kawasaki 216, Osaka, Japan
4. Division of Veterinary Microbiology, Kyoto-Biken Laboratories Inc., 24-16 Makishima-cho, Uji 611, Osaka, Japan