Mice, carriers of the Ehrlich ascites carcinoma, were exposed to a fractionated whole-body irradiation with four times 750 R. Before resp. after each exposure, an ozone-oxygen mixture was intraperitoneally injected. In other cases, the ozone-oxygen mixture was given without irradiation. Ozone had no essential effect upon the transplantation capacity neither in irradiated ascites cells nor in cells not exposed to radiation. Ehrlich ascites tumor cells, transplanted into a subcutaneous air-pocket under the abdominal skin of the mouse, developed a solid tumor and a fluid accumulation without cells. A diminution of the fluid was brought about by means of a fractionated local irradiation with five times 200R, by a secondary treatment with ozone and also by ozone alone. Evidence of damage to the tumor cells, however, is only to be produced by investigations into the metabolism and reproductive capacity of these cells.