The sensitivity of 37 solid tumours of children was tested in vitro towards cytostatic agents by means of an autoradiographic short-term method. Sensitivity was measured as the magnitude of inhibition of 3 H-thymidine incorporation. The test was performed with the cytotoxic agents Cyclophosphamide, Trenimon, Bleomycin, Adriamycin, Daunomycin, Actinomycin D, and Cytosin-Arabinosid in 9 Wilms' tumours, 9 neuroblastomas, 7 non-Hodgkin-lymphomas, 5 osteogenic sarcomas, 3 soft tissue sarcomas, and 4 special tumours. None of the tumours is resistant to all cytotoxic substances. The tumours show a marked individual sensitivity pattern, and, with few exceptions, they are sensitive against 2 or more cytostatics. This behaviour is explained mainly by the usually high proliferative activity of dysontogenetic tumours, malignant lymphomas and various sarcomas. The possibilities and limits of the short-term methods for sensitivity-testing are discussed critically and in detail. For the evaluation of the results of in vitro testing and of in vivo effectiveness the close coreelations are not always taken into consideration between the type of cytostatic agent and effect on tumour metabolism, cytostatic agent and proliferation kinetics of the tumour as well as the effect of the cytostatics and the nucleic acid precursor used for the test. Despite the methodological limitations preclinical testing should be preferred in comparison with unselected chemotherapy.