Natural Killer cells are considerd to play an important role in antitumor immune surveilance mechanism. In this study, 21 putative anticancer drugs selected from reference were assessed by evaluating the effect on rat Natural Killer cell activity (NKCA). All 21 herb drugs were extracted in boiling water, lyophilized, autoclaved, and then used for experiment. Culture supernatant of concanavalin-A (Con-A)-stimulated rat spleen cells as a source of lymphokine was also used as a control of comparison. Rat spleen cells were used as effector and NKCA was measured in 4hr $^{51}Cr-release$ assay against Yac-1 mouse lymphoma cell line. In order to determine the optimal conditions for NKCA augmentation, effector cells were treated with 3 different concentrations of each drug for 24, or 48 hrs before testing of NKCA, In optimal conditions determined from previous results, the effect of herb drugs on NKCA were assessed in 3 to 5experiments. NKCA was significantly enhanced by treatment with 4 herb drugs(Ponciri Fructus, Houttuyniae Herba, Aurantii Pericarpium, Nepetae Herba). Culture supernatant of Con-A-stimulated spleen cells also augmented the rat NKCA more significantly. The results show that 4 of the herb medicines supposed to display anticancer effect may have activity as a biological response modifier through augmentation of NKCA.