Abstract: The effects of ribvirin(1-β-D-ribofuranosy 1-1,2,4-triazole-3-carboxamide,RTC)on natural killer(NK)activities of murine spleen cells in vitro were studied by¹²⁵I-UdR isotoperelease assay.The results showed that RTC at the concentration of 5 umol·L^-1 significantlysuppressed NK activities(P<0.01),and a negative correlation between NK activities and RTCdosage was obtained.It also indicated that there was additive effect between the actions of RTC anddexamethasone(Dex,1 umol·L^-1 ),and antagonism between the actions of RTC and interferon-γ(100 U·ml^-1 ).In addition,RTC at the concentration of 0.5 umol·L^-1 was found to be enough toreduce the product of interferon(P<0.01)and interieukin-2(P<0.05)induced by Con A in murinespleen cells,and a negative dcoe-effect relationship was obeerved.Thus,one of the mechanisms ofRTC in inhibiting NK activities seems to suppress the production of these lymphokines.