Human blood plasma and freshly isolated LDL were exposed to singlet oxygen (1O2) by thermal decomposition of synthetic endoperoxides. Exposure of blood plasma to 20 mM water-soluble 1O2 generator resulted in the depletion of ascorbate (100%), urate (75%), ubiquinol-10 (65%), protein thiols (50%), and bilirubin (25%), whereas under these conditions the levels of alpha-tocopherol, beta-carotene, and lycopene remained unchanged. The following rates of depletion were obtained by kinetic analysis (moles depleted per 100 mol of 1O2 consumed): protein thiols (5), urate (5), ascorbate (4), bilirubin (1), and ubiquinol-10 (0.008). In contrast, the rates of depletion using the lipid-soluble 1O2 generator were faster for bilirubin (13-fold), protein thiols (9-fold), ubiquinol-10 (8-fold), and ascorbate (5-fold), and slower for urate (2-fold). The formation of lipid hydroperoxides, including mostly cholesteryl linoleate hydroperoxide, was observed in 1O2-treated plasma (0.007-0.009 mol/100 mol 1O2) and LDL solutions (0.086 mol/100 mol 1O2). Based on competition kinetics, we estimate that 98% of 1O2 generated in the aqueous phase of plasma is quenched by components in this phase, mostly by plasma protein (63%; 6% by protein thiols), urate (9%; 5% by chemical quenching), and bilirubin (5%; 1% by chemical quenching). Ascorbate and ubiquinol-10 do not contribute to 1O2 quenching in plasma, and their oxidation is probably mediated secondary species. The remaining 1O2 generated in plasma (2%) diffuses into lipoprotein leading to the formation of lipid hydroperoxides with an efficiency of about 100-fold greater than that compared to aqueous generated 1O2. The principal 1O2 quenchers in LDL include apoB (42%), lycopene and beta-carotene (40%), and alpha-tocopherol (17%). The importance of carotenoids in the quenching of 1O2 in lipoprotein suggest that the beneficial effects of these compounds in health may in part be due to the elimination of this species in biology and medicine.