The objective of the present study was to investigate the beneficial properties of magnoflorine, an alkaloid isolated from coptidis rhizoma, on protecting human high density lipoprotein (HDL) against lipid peroxidation. Magnoflorine exerts an inhibitory effect against Cu2+-induced lipid peroxidation of HDL, as showed by prolongation of lag time from 62 to 123 min at the concentration of 3.0 microM. It also inhibits the generation of thiobarbituric acid reactive substances (TBARS) in the dose-dependent manners with IC50 values of 2.3+/-0.2 microM and 6.2+/-0.5 microM since HDL oxidation mediated by either catalytic Cu2+ or thermo-labile radical initiator (AAPH), respectively. Separately, Cu2+ oxidized HDL lost the antioxidant action but the inclusion of magnoflorine/Cu2+ oxidized HDL can protect LDL oxidation according to increasing magnoflorine concentration. The results suggest that magnoflorine may have a role to play in preventing the HDL oxidation.