Antibiotics have been displaying their excellent efficacy in the treatment of bacterial infections. More recently it was noticed that some antibiotics would be applicable also to malignant tumors. As a result of a series of screening studies carried out in our laboratory, several antibiotics with antitumor activities were found. They are luteomycin^1,2). carzinophilin^3,4), mitomycin^5,6) and melanomycin⁷⁾. The author had an opportunity to study the cytomorphological activities of these substances on ascites tumor cells and to compare them with these of known antitumor agents, such as sarkomycin⁸⁾, nitromin (nitrogen mustard N oxide) ⁹⁾ and azan M (8-azaguanine methanesulfonic acid-Na) ¹⁰⁾.

At present, various transplantable tumors are being employed in various kinds of systems for studies of chemotherapy. For evaluating the antitumor activities, cytomorphological effects of agents on ascites tumor cells would be conceived to have no less important back ground than their effect on inhibition of tumor growth or prolongation of survival of host animals.

The first report of cytomorphological change caused by antitumor substance on ascites tumor was published by Schairer¹¹⁾ and Lettré¹²⁾ using colchicine. Since discovery of Yoshida sarcoma by Yoshida in 1943, many works with various agents on the tumor have been reported. For instance, colchicine was studied by Satō¹³⁾, Hirono¹⁴⁾ and others; nitromin by Ishidate and Yoshida^9,15), Kaise^16,17), Tokuyama¹⁸⁾ and Ohboshi, et al^19,20); sarkomycin by Yamamoto²¹⁾ and Umezawa, et al.²²⁾, and 8-azaguanine by Kondō²³⁾, Yamamoto²⁴⁾ and Aizawa²⁵⁾.

In searching the antitumor activity of various substances, the author observed cytomorphological changes of tumor cells and other cellular elements in the ascitic fluid of tumor-bearing animals after administration of the substances mentioned above.