Hypochlorous acid (HOCI) is a powerful oxidizing and chlorinating agent produced by the neutrophil enzyme myeloperoxidase. The antioxidant defenses of freshly prepared human plasma against HOCI/OCI- were explored. Addition of HOCI/OCI- to plasma caused rapid oxidation of ascorbic acid and thiol (-SH) groups but not of uric acid. Plasma -SH groups (which are known to be largely located on albumin) were quantitatively the most important scavenger of HOCI/OCI-, but adding extra ascorbate to plasma caused this molecule to have a more important scavenging role against HOCI/OCI-. Added HOCI/OCI- produced no detectable lipid peroxidation in plasma or depletion of lipid-soluble antioxidants (alpha-tocopherol or ubiquinol-10). No evidence of oxidative damage to protein amino acid residues (other than -SH) was detected by the carbonyl assay. It seems that -SH groups are a major target of attack by HOCI/OCI- in vivo, and plasma albumin may be an important protective antioxidant. Ascorbic acid might also play a protective role, especially in individuals supplemented with this vitamin. Ascorbate might also be important in extracellular fluids with low albumin concentrations such as synovial, respiratory tract lining, and cerebrospinal fluids.