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