Coix seeds, a Chinese medicine have been used in Japan and reported to be effective in patients with verruca vulgaris and verrucae planae juveniles. We investigated the in vivo effects on lymphocyte subsets in seven healthy volunteers who took six tablets of Coix seeds three times a day (a typical dose) for four weeks. Leukocyte counts and the percentage of total lymphocytes did not change but the percentages of CD3+CD56+ cells and CD16+CD57- cells increased significantly. These results indicate that Coix seeds increase peripheral cytotoxic lymphocytes and may be effective to viral infection through the enhancement of cytotoxic activity.