Human serum albumin (HSA) reduced the phospholipid hydroperoxide, 1-palmitoyl-2-(13-hydroperoxy-cis-9, trans-11-octadecadienoyl)-l-3-phosphatidylcholine (PLPC-OOH) to the corresponding hydroxy-derivative with a high apparent affinity (Km=9. 23+/-0.95 microM). Removal of bound lipid during purification increased this activity. At physiological concentration, HSA reduced the phospholipid hydroperoxide in the absence of a cofactor. However, in the presence of a cofactor (reductant), the rate of the reaction was increased. All of the major aminothiols in plasma could act as reductants, the best being the most abundant, cysteine (Km=600+/-80 microM). For every nanomole of PLPC-OOH reduced by HSA, 1.26 nmol of cystine was formed, indicating a reaction stoichiometry of 1 mol PLPC-OOH to 2 mol cysteine. We used chemical modification to determine which amino acid residues on HSA were responsible for the activity. Oxidation of thiol group(s) by N-ethylmaleimide led to a reduction in the rate of activity, whereas reduction of thiols by either dithiothreitol or the angiotensin-converting enzyme inhibitor, captopril, increased the activity. Both N-ethylmaleimide-modified HSA and dithiothreitol-treated HSA exhibited increased apparent affinities for PLPC-OOH. For a range of preparations of albumin with different modifications, the activity on PLPC-OOH was dependent on the amount of free thiol groups on the albumin (correlation coefficient=0.91). Patients with lowered albumin concentrations after septic shock showed lowered total plasma thiol concentrations and decreased phospholipid hydroperoxide cysteine peroxidase (PHCPx) activities. These results therefore show for the first time that HSA exhibits PHCPx activity, and that the majority of the activity depends on the presence of reduced thiol group(s) on the albumin.