In a pilot study of 40 patients with inoperable bronchiogenic carcinomas the results of clinical therapy were compared with those of in vitro tests by measuring the effects of cytostatic agents on the incorporation of precursors into DNA and RNA in tumour cells incubated in vitro for short periods. Tumour cells that showed little inhibition by the cytostatic agents in vitro (less than 30% inhibition at a concentration of 10(-2) mg/ml adriamycin) came from patients who responded poorly to cytostatic therapy. On the other hand, tumour cells that were strongly inhibited in vitro (greater than 30% inhibition) came from tumours which proved to be sensitive to the chemotherapy given to the patient. A strong correlation existed between the inhibition of uridine incorporation by the tumour cells in vitro by adriamycin and the responses of the tumour to clinical therapy even when combination-therapy was given without adriamycin. There was also a significant correlation between the inhibiting effect of adriamycin in vitro and the survival time of the tumour patients.