This study investigates the temporal disappearance of endogenous antioxidants in human blood plasma and the appearance of lipid hydroperoxides under two types of oxidizing conditions: exposure to aqueous peroxyl radicals and stimulation of polymorphonuclear leukocytes (PMNs). The results show that exposure to aqueous peroxyl radicals leads to immediate oxidation of ascorbate and sulphydryl groups, followed by sequential depletion of bilirubin, urate, and α-tocopherol. Stimulation of PMNs results in rapid oxidation of ascorbate and partial depletion of urate, followed by the formation of micromolar concentrations of hydroperoxides of plasma phospholipids, triglycerides, and cholesterol esters. Nonesterified fatty acids, bound to albumin, are better protected from peroxidative damage than esterified fatty acids. The study also reveals that albumin-bound bilirubin and albumin itself provide site-specific antioxidant protection. The findings suggest that ascorbate plays a crucial role in protecting lipids from peroxidative damage and that controlled ascorbate supplementation could be beneficial in preventing lipid hydroperoxide formation, which is implicated in various diseases associated with oxidative stress.This study investigates the temporal disappearance of endogenous antioxidants in human blood plasma and the appearance of lipid hydroperoxides under two types of oxidizing conditions: exposure to aqueous peroxyl radicals and stimulation of polymorphonuclear leukocytes (PMNs). The results show that exposure to aqueous peroxyl radicals leads to immediate oxidation of ascorbate and sulphydryl groups, followed by sequential depletion of bilirubin, urate, and α-tocopherol. Stimulation of PMNs results in rapid oxidation of ascorbate and partial depletion of urate, followed by the formation of micromolar concentrations of hydroperoxides of plasma phospholipids, triglycerides, and cholesterol esters. Nonesterified fatty acids, bound to albumin, are better protected from peroxidative damage than esterified fatty acids. The study also reveals that albumin-bound bilirubin and albumin itself provide site-specific antioxidant protection. The findings suggest that ascorbate plays a crucial role in protecting lipids from peroxidative damage and that controlled ascorbate supplementation could be beneficial in preventing lipid hydroperoxide formation, which is implicated in various diseases associated with oxidative stress.