1. Selenium is an essential component of glutathione peroxidase (GSH-Px, EC 188.8.131.52), an enzyme which helps protects cells against damage caused by free radicals and hydroperoxides. 2. We report the plasma, whole blood and platelet concentrations of selenium, and whole blood and platelet activities of GSH-Px, in 49 patients with asthma, 23 of whom had coexisting eczema, and 76 healthy control subjects. 3. The asthmatic patients had significantly lower concentrations of selenium measured in plasma (P less than 0.001) and whole blood (P less than 0.001), but not in platelets. When the data were summarized as odds ratios there was a highly significant 3.54- and 5.08-fold increased probability of asthma observed for the lower range of plasma and whole blood selenium concentrations, respectively. 4. No overall decrease in platelet or whole blood GSH-Px activity was found when the asthmatic and control groups were compared. 5. Although patients with symptomatic asthma have a reduced selenium status, this does not appear to influence the antioxidant capacity of their circulating blood cells.