Bovine superoxide dismutase (SOD) (Peroxinorm, Grünenthal Stolberg) was injected intramuscularly or subcutaneously in a daily dose of 4 mg over a period of six weeks into two patients with Fanconi anaemia. The effects (measured by the decrease of chromosome aberrations in blood lymphocytes and the increase in blood cells in venous blood) were evident but temporary. The hypothetical mode of action of SOD and the failure of a prolonged therapeutic effect are discussed.