Abstract: The effects of harringtonine on nucleotide metabolism in Ehrlich ascites carcinoma cells have been studied. While the incorporation of ³H-TdR into the acid-soluble fraction was enhanced, distinctly by harringtonine, the DNA synthesis was inhibited. The increase of ³H-dTTP was most pronounced among the thymine nucleotides(dTMP,dTDPand dTTP). However, the incorporation of ³H-TdR into each of these nucleotides in the acid soluble fraction was the same in the harringtonine-treated cancer cells as that in the control. This suggests that harringtonine might block the polymerization of dNTP into DNA, but without influence on the phosphorylation of TdR.It was also noted that harringtonine showed no significant effect on the incorporation of ³H-UR and ³H-HX into the nucleotide triphosphates and on RNA synthesis. These results together with the data of NTP and dNTP analysis shown by us previou sly suggest that the reduction of NDP seemed not to be inhibited, by the drug.