Coix seed has been used in patients with verruca vulgaris and verruca planae juveniles, which have been considered to be induced by viral infection. Moreover, coixenolide, component in the seeds of coix, was reported to show anti-tumor activity. Possibly coix seed may have some influence on the cytotoxic activity of peripheral lymphocytes but there has been no data on this. Then we investigated the changes in number of cytotoxic lymphoid cells in seven volunteers before, during (four weeks) and after taking six coix seed tablets. Lymphocyte subsets were analyzed with monoclonal antibodies using a flow cytometer. The level of CD3+CD56+ (MHC-non restricted cytotoxic T cells) markedly increased at four weeks (before 1.9 +/- 0.5% vs four weeks 4.2 +/- 0.7%, p less than 0.01). The level of CD16+CD57- (the mature, most active natural killer cells) increased at three weeks (before 4.5 +/- 0.8% vs three weeks 5.2 +/- 0.8%, p less than 0.05). The level of CD3-CD56+ (natural killer cells) and the level of CD16+CD57+ (the variable active natural killer cells) decreased at one week and returned to normal level thereafter (before 13.7 +/- 2.1% vs one week 11.2 +/- 1.5%, p less than 0.05; before 8.8 +/- 1.5% vs one week 6.9 +/- 1.3%, p less than 0.05, respectively). These results indicate that coix seed modulate the peripheral blood lymphocyte subsets and may be effective to virus disease through the enhancement of cytotoxic activity.