The increases in radiobiological hypoxic fraction of RIF-1 and KHT tumours 30 min after 70 mg kg-1 BW12C i.v. were measured and shown to be similar; factors (+/- 2 s.e.)ranged from 3.87 (2.84-5.29) to 5.92 (1.92-18.2) despite the large variation in initial hypoxic fraction, from 0.30 (0.18-0.50) % for RIF-1 intramuscularly in the leg to 16.3 (14.7-18.1) % for subcutaneous KHT flank tumours. The gene discussed is RIF1; the disease is neoplasm.