The present study was performed to investigate plasma inhibitors of endothelium-dependent relaxation other than hemoglobin and low-density lipoprotein (LDL). We purified an inhibitor that contained a protein of 28,000 Da from human plasma by ammonium sulfate precipitation and serial chromatography. NH2-terminal sequence analysis revealed the protein to be homologous with human apolipoprotein A-I (Apo A-I), a major apolipoprotein of high-density lipoprotein (HDL). Very low-density lipoprotein (VLDL), LDL, and HDL obtained from rabbit plasma reversed endothelium-dependent relaxation of rabbit aorta induced by acetylcholine (ACh) and A23187 but did not inhibit relaxations induced by nitroglycerin or nitric oxide. These inhibitory activities were lost by delipidation of lipoproteins, and there were no differences in the inhibitory activity among these three lipoproteins on the basis of phospholipid concentration. Moreover, phospholipids such as phosphatidylcholine, phosphatidylinositol, and sphingomyelin reversed relaxations by ACh and A23187. Thus all lipoproteins inhibit endothelium-dependent relaxation, and this nonspecific inhibition seems to be due to the inhibition of production or release of endothelium-derived relaxing factor by phospholipids in the lipoprotein complex.