From various new anthracylines containing fluorine in their sugar moieties, 7-0-(2,6-dideoxy- 2-fluoro-e-1-talopyranosyl)-adriamycinone-l4-0-alaninate HC10A-125) has been selected for ciinicai investigation, because of its excellent antitumor acitivity and low toxicity. In this paper the mechanism whereby DA-125 exhibits good antitumor activity in viw was studied through in vitro experiments in comparison with doxorubicin(DXR). DA-125 had stronger activity than DXR in single and double strand scission in P388 cells and in inhibition of nucleic acid synthesis in L1210 cells. But DA-125 had weaker activity than DXR in binding to calf thymus DNA. DA-125 was taken up by P388 cells faster than DXR and accumulated reaching at 2. 7 fold higher leveL