Hiromi Kohno
Hirofumi Sakai
Yuzaburo Oku
Masao Kamiya
Department of animal disease control, Graduate School of Veterinary Medicine, Hokkaido University, Sapporo 060, Japan.

Munehiro Okamoto
The Institute of Experimental Animal Science, Osaka University Medical School, Suita-shi 565, Japan.

Mamoru Ito
Central Laboratory for Experimental Animals, Kawasaki, Japan.