Abstract: A simple and rapid method is described whereby short-term suspension culture of Yoshida ascites sarcoma cells may be used as a screening tool for anti-tumor substances. The tumor cells were cultivated in a medium consisting of horse serum and Hanks' balanced salt solution; sixty thousand to one hundred thousand cells per ml were used as the initial inocula. Cell-counts were carried out at 24 and 48 hours after addition of the drug and the record of the effect of the drugs was graded mainly with reference to the per cent inhibition of the tumor cells. In addition, cytomorphological changes were also noted as complementary criteria. The response of this test system was assessed first by exposing it to five known anti-tumor agents, and it was observed that these compounds exhibited a varying antitumor activity against the cultured cells. In decreasing order of effectiveness they were listed as follows: HN₂, nitromin, novoembichinum, myleran and 6-mercaptopurine. Moreover, 117 drug preparations had been screened with this technique. The results indicated that 41.1% of the 24 synthetic compounds and 38.7% of the 93 extracts of natural medicines were found to be active. As regards the comparison of these in vitro results with the in vivo antitumor effects of the same substances in tumor-bearing animals, it was pointed out that there was some, though not perfect, correlation between these two biological test systems. A discussion on the usefulness of this technique as a large scale cancer chemotherapy screening procedure had been made, and its limitations were also presented.