The chemotherapeutic activity of ifosfamide, applied i.v. at a high single dose, was tested on some transplantable tumours of BD-rats. As standard substance cyclophosphamide was used. The margin of safety of ifosfamide in DS-carcinosarcoma and TA-nephroblastoma was found to be wider than that of cyclophosphamide. Therefore, using massive single doses of ifosfamide, rats could be cured of DS-carcinosarcoma in higher percentage than by an analogous treatment with cyclophosphamide. Further, with a single i.v. dose of 280 mg/kg ifosfamide was it possible to cure all TA-nephroblastoma-bearing animals. The ANH-nephroblastoma, however, could not be cured even with the highest tolerable doses of ifosfamide. It was, nevertheless, possible to attain higher increase of life span over the controls than by cyclophosphamide-treatment. In some other isologous tumours ifosfamide proved to be less effective or without any activity, respectively. Pretreatment by repeated small doses of ifosfamide resulted in resistance of DS-carcinosarcoma against curative action of subsequent high single doses of this drug.