The purpose of this research was to investigate the effects of Coix Lachryma-Jobi L. var. Ma-yuen Stapf. on the proliferation of L1210 cells and immune cells. The hexane fraction of Coix Lachryma-Jobi decreased the proliferation of L1210 cells and induced DNA fragmentation of L1210 cells. Also, the fraction decreased the cell viability of murine thymocytes and splenocytes, but increased the phagocytic activity of murine peritoneal macrophages. In addition, fatty acids and fatty acid methyl esters decreased the proliferation of L1210 cells and induced DNA fragmentation of L1210 cells. The main components of the fraction are fatty acid and their derivatives. These results indicate that Coix Lachryma-Jobi has a cytotoxicity on tumor cells and immune cells.