The effects of Buthus martensi Karsch (BMK) on natural killer (NK) cell activity in mouse spleen were studied. Water extracted solution of BMK was orally administrated to Balb/c mice for 2 weeks. Among splenic cells, T cell fractions were separated by Nylon wool column. Furthermore, NK cell purification was performed 4.5% percoll gradients methods. The cytotoxcity of NK cell to K562 cell was determined by lactic acid dehydrogenase and $[^3H]-thymidine $ incorporation methods. And the cytotoxicity of effector cell was most effectively induced in a ration of 50:1 (effector/target cell). As a result, cytotoxicity of NK cells was significantly increased compared with control group both in vivo and in vitro systems. The similar cytotoxic effect was shown in $[^3H]-thymidine $ incorporation methods. This suggests that when BMK is administrated to mice with malignant tumors, an increase in NK cell activity may occur and affect K562 tumor cells.