The hypoxic cell radiosensitizer, Ro 07-0582, has now been given in multiple doses to 16 patients. They have received a total of 15-51 g in 3-20 doses. Immediate tolerance was good, and satisfactory plasma levels of the drug were consistently obtained. Neurotoxicity was, however, troublesome: convulsions occurred in the patient given the highest dose, and there was peripheral neuropathy in 11 cases. Tumour concentrations similar to those in plasma were obtained in human tumours, in contrast to the findings in mouse tumours where concentrations are usually below 40% of plasma levels. In the treatment of human tumours, a lower dose of Ro 07-0582 should give useful hypoxic cell sensitization. Although the total dose of Ro 07-0582 must be limited, there is a real prospect that it will give benefit in clinical radiotherapy. Images Fig. 4