To determine phosphatidylcholine hydroperoxide (PCOOH) in human plasma lipids, a chemiluminescence -high performance liquid chromatography (CL-HPLC) system was newly developed and applied. This system * To whom correspondence should be addressed. consisted of separation of phosphatidylcholine derivatives with normal phase HPLC and detection of hydroperoxide-specific chemiluminescence of PCOOH. By use of this system, the PCOOH in normal human plasma was quantitatively assayed to be 28-431 pmol hydroperoxide-02 per ml of plasma. The high sensitivities, reproducibilities and specificities of the CL-HPLC assay were shown for determining plasma PCOOH.