1. Toxicology Division, Mitsukaido Laboratories, Institute of Environmental Toxicology, Uchimoriya-cho 4321, Mitsukaido-shi, Ibaraki 302-02, Japan
2. Department of Laboratory Animal Science, Nippon Veterinary and Zootechnical College, Kyonan-cho 1-7-1, Musashino-shi, Tokyo 180, Japan