The reaction of ozone (0,) with human blood plasma was studied to help understand possibleevcnts that could occur in the respiratory trtit. Uric acid (quantitatively the most important scavenger) and ascorbic acid were oxidized quickly, protcin.SH groups were lost more slowly, and there was no loss or bilirubin or a-tocopherol. There was little formation of lipid hydroperoxides and no detectable formation of Chydroxynoncnal, hexanal or nonanal, or changes in lipoprotein clcrtrophoretic mobility. Uric acid in human upper airway secretions may play a significant role in removing inhaled 0,. Oxidative damage to lipids must not bc assumed to be the key mechanism of mrpiratory tract 0, toxicity.