The preliminary details of a randomized clinical trial of misonidazole for the radiotherapy of Grades 3 and 4 cerebral astrocytoma are described. Plasma concentrations of misonidazole and its O-demethylated metabolite, determined by high-pressure liquid chromatography analysis, are reported for 8 patients with astrocytoma, 1 with carcinoma of the bronchus and 1 with carcinoma of the breast. In the latter case, tumour concentrations are also presented.