Sodium warfarin was studied for its in vitro effect оп nuclei acid synthesis of L121О leukcemic cells and Walker 256 carcinosarconia cells. Moderate inhibition was noted in the incorporation of thymidine and uridine into DNA and RNA in L1210 cells, Ьut only at very high concentrations, and no inhibition was seen in Walker tumor cells. The survival of LI 210-bearing mice was not influenced by various regimens of sodium warfarin treatment. These data indicate that sodium warfarin probably has no direct in vivo antitumor activity against LI 210 leukemia of mice or Walker 256 carcinosarcoma of rats.