The clinical application of cytostatic drugs requires knowledge of their biochemistry, pharmacology, pharmacokinetics as well as their action at the cellular level within the generation cycle. The principles apply to tumor cells as well as to normal, rapidly proliferating tissues. The intensive research on cancer treatment all over the world is leading to a rapid accumulation of experimental data about the action of single cytostatic drugs in tissue culture and on transplantable animal tumor systems, especially in rodents. Clinical chemotherapy in human malignancies today preferentially uses combinations of different cytostatics. Inumerable combinations of drugs are available, especially if variations in respect to drug dose and intervals of drug application are taken into consideration. The experimental basis for such combinations of drugs and drug interactions is scanty. Using the pyrimidine analog cytosine arabinoside and the two antibiotics daunorubicin and doxyrubicin (adriamycin) as examples, it is demonstrated that information on the pharmacolinetic behaviour of cytostatic drugs is a prerequisite for their success in clinical application, but is on its own insufficient to predict the tumor response.