Abstract: The mode of action of drugs that inhibit DNA synthesis can be classified into two categories; one is to damage the DNA template, the other is to interfere with DNA bio-synthesis. Following up the changes of the rate of DNA synthesis in Ehrlich ascites cells after exposing to the drugs that inhibit DNA synthesis of these tumor cells, one can easily determine the mode of action of the types of inhibition. Thus a decreasing curve of ³H-thymidine incorporation into DNA would suggest a type of mode of action through damage of DNA template, while an ascending curve would suggest one through interference of DNA metabolism. With this method, it was shown that antitumor drugs such as nitrogen must-ard, bleomycin and daunorubicin exerted their inhibitory action on DNA synthesis of tumor cells through damage of DNA template. On the other hand, the inhi-bitory action of 5-fluorouracil, cytosine arabinose and methotrexate on DNA synthesis was through interference of DNA metabolism. These results were in agreement with those determined by other methods.