Normal human plasma contains numerous high- and low-molecular-mass redox-active molecules that are able to react rapidly with organic and inorganic oxygen radicals. The ability of such plasma molecules to substantially inhibit, or delay, free-radical mediated oxidation of added substrates has led to their classification as important biological antioxidants. Using phospholipids to detect organic oxygen radicals and deoxyribose to detect inorganic oxygen radicals, we here show that the primary antioxidants of normal human plasma reside mainly in two plasma proteins representing no more than 4% of the total proteins present. The iron-binding properties of transferrin and the iron-oxidising properties of caeruloplasmin, at a reaction dilution of 1:50, offer considerable protection against organic and inorganic oxygen radicals generated by iron and ascorbate. Plasma thiol-group-containing molecules, at concentrations well below those that would be required to compete with the detector molecule (based on known second-order rate constants for reaction with hydroxyl radicals) inhibited damage to deoxyribose, but stimulated damage to phospholipids.