A polyphenol oxidase in Tsuyama “jinenjo” (Japanese yam) showed the highest activity under the conditions of pH 6.6 at 40°C. Pyrocatechol was found to be the most suitable substrate for this enzyme from the point of browning. The major polyphenol in jinenjo was pyrocatechol. Annual changes in the polyphenol oxidase and hemagglutinating activities were studied. Both activities showed seasonal changes with a peak in October, the pattern of these seasonal activity changes being similar. The best harvest time for jinenjo was judged to be later than November from the most desirable cooking condition of its “whiteness” due to the low activity of polyphenol oxidase. This harvest time was also appropriate, because jinenjo, which is often eaten raw, contained a low level of lectin and showed low toxicity.