3-Chloro-L-Tyrosine (Chlorotyrosine), a specific marker of myeloperoxidase-catalyzed oxidation, is markedly elevated in low-density lipoprotein isolated from human atherosclerotic intima. In particular, myeloperoxidase halogenates tyrosine residues in plasma proteins and generates 3-chlorotyrosine (CY). The detection of free chlorotyrosine in blood or urine arises from the degradation of these chlorinated proteins. CY concentrations may be useful for monitoring the activation of neutrophils in asthmatic patients.