1. Iron-overloaded spleens from thalassaemic subjects showed a greatly increased susceptibility to free-radical oxidation (peroxidation) in vitro.
2. Iron content was the main but not the only variable governing susceptibility. Added ascorbate had a dose-dependent action either as an antioxidant or as a pro-oxidant, the direction of its effect depending mainly on the degree of iron overload. This has therapeutic implications.