Several studies have shown that extracts from yellow-green vegetables reveal antitumor activities. In the present study we investigated the effect of phytol in order to elucidate the immunological mechanism of antitumor activity of this substance. The results obtained from the experiment as follows: 1) Phytol showed cytotoxic effect on sarcoma 180 cells in vitro. 2) When phytol was injected into the peritoneal cavity of mice transplanted with sarcoma 180 cells, the average survival time (24.0 days) tended to increase as compared with the nontreated control (19.2 days). 3) When sarcoma 180 cells were injected subcutaneously into the right groin of mice, and then phytol was injected into the peritoneal cavity, the tumor inhibition ratio was 33%. 4) The natural killer(NK) cell activity was significantly augmented by phytol in vitro and in vivo. Similar augmentations of NK cell activity were obtained with culture supernatants of phytol exposed spleen cells and peripheral blood mononuiclear cells. 5) Phytol on the macrophage from peritoneal cavity showed a higher effectiveness in vivo than in vitro. These results indicate that phytol shows the inhibitory effect for growth of sarcoma 180 cells in vitro, also it can augment macrophage and NK cell activities in vivo.