Oxygen-derived free radical damage is associated with the molecular toxicity of hemoglobin. Especially in thalassemia syndromes, this toxicity has a relationship with "free" alpha globin concentrations. This study of beta thalassemia trait blood samples from 39 individuals shows that the evaluation of methemoglobin is a sensitive method of indicating molecular toxicity and the superoxide dismutase concentration revealing the intensity of oxidative stress of this process.