The tumour used, designated MT1, is a more radiosensitive form of the anaplastic MT tumour previously described. No explanation for the increased radiosensitivity was found, but it was shown not to be due to infection or to a change in immunological status, growth rate or histology. The sensitivity has remained constant throughout the present work. No cytotoxicity in the tumour was observed when 1 mg/g body weight of Ro-07-0582 was injected immediately after a single dose of X-rays; indeed a small protective effect was seen. A radiosensitization enhancement of 1-5 was achieved with a relatively low drug dose of Ro-07-0582 in a 5F/4d fractionated regime. The interval between the injection of a low dose of Ro-07-0582 and the start of irradiation was found to be critical, the optimum interval being 45-60 min. The subsequent incidence of distant metastases was not increased by the use of Ro-07-0582 at the time of "primary" tumour irradiation.