Hypohalous acids are generated from the oxidation of halide ions by myeloperoxidase and eosinophil peroxidase in the presence of H2O2. These oxidants are potent antibacterial agents, but excessive production can result in host tissue damage, with this implicated in a number of human pathologies. Rate constants for HOCl with lipid components and antioxidants have been established. Here, the corresponding reactions of HOBr have been examined to determine whether this species shows similar reactivity. The second-order rate constants for the reaction of HOBr with 3-pentenoic acid and sorbate, models of unsaturated lipids, are 1.1x10(4) and 1.3x10(3) M(-1) s(-1), respectively, while those for reaction of HOBr with phosphoryl-serine and phosphoryl-ethanolamine are ca. 10(6) M(-1) s(-1). The second-order rate constants (M(-1) s(-1)) for reactions of HOBr with Trolox (6.4x10(4)), hydroquinone (2.4x10(5)), and ubiquinol-0 (2.5x10(6)) were determined, as models of the lipid-soluble antioxidants, alpha-tocopherol, and ubiquinol-10; all of these rate constants are ca. 50-2000-fold greater than for HOCl. In contrast, the second-order rate constants for the reaction of HOBr with the water-soluble antioxidants, ascorbate and urate, are ca. 10(6) M(-1) s(-1) and closer in magnitude to those for HOCl. Kinetic models have been developed to predict the sites of HOBr attack on low-density lipoproteins. The data obtained indicate that HOBr reacts to a much greater extent with fatty acid side chains and lipid-soluble antioxidants than HOCl; this has important implications for HOBr-mediated damage to cells and lipoproteins.