[en] The compound Ro 05-9963 is a metabolite of misonidazole (Ro 07-0582; 1-(2-nitroimidazol-l-yl)-3-methoxypropan-2-ol) formed by 0-demethylation. Radiosensitization of the anaplastic tumour MT1 in WHT/Ht mice by Ro 05-9963 was determined from local tumour control at 80 days after single doses of 240kV X-rays. Two Chinese hamster lung cell lines were used for in vitro radiosensitization, and hypoxic and aerobic toxicity studies. The results show that Ro 05-9963 is an efficient radiosensitizer in its own right and may contribute to the in vivo enhancement of tumour cure by misonidazole. Furthermore, the cytotoxic effect known to be produced by misondazole in vitro could probably also be usefully supplemented by the presence of Ro 05-9963 in tumours. (U.K.)