3H-thymidine and 3H-uridine labelling indices (LIth and LIur) were determined simultaneously using an autoradiographic method in 19 solid tumours: 6 Wilms' tumours, 5 neuroblastomas, 4 osteogenic sarcomas, and 4 various tumours. At a LIth of 2.7 to 47.0% (average 22.3%) all LIur were significantly higher with values of 11.6 to 76.3% (average 47.4%). The difference between both indices varies by the factor 1.26 to 7.45 (average 2.83). The magnitude of 3H-uridine incorporation into the RNA is correlated to the proliferation rate as well as that of 3H-thymidine incorporation into the DNA. Rapidly growing tumours show high 3H-thymidine and -uridine labelling indices with a decrease from Wilms' tumours to osteogenic sarcomas and neuroblastomas. 3H-thymidine and -uridine incorporation into nucleic acids are discussed with reference to the cell kinetic system of a solid tumour and the course of the cell cycle. Perhaps, the 3H-uridine labelling index in the autoradiograms represents the growth fraction of a tumour. The significance of the relation of DNA to RNA synthesis for the testing of tumour sensitivity towards cytostatic agents in the short-term test is discussed.