1. Division of Food, National Institute of Health Sciences, 18-1 Kamiyoga 1-chome, Setagaya-ku, Tokyo 158, Japan, National Research Institute of Fisheries Sciences, 31-1 Nagai, 6-chome, Yokosuka, Kanagawa 238-03, Japan, and Hokkaido Institute of Public Health, North 19, West 12, North Ward, Sapporo 060, Japan