Lipid peroxidation in human LDL (0.05 mg protein/mL) incubated with Cu(2+)-ions (5 microM) in vitro was dose-dependently inhibited by the flavonoid myrigalone B (MyB) and by ascorbic acid. MyB at 6 microM increased the oxidation lag time by 135 +/- 24 min (approximately 5-fold compared to controls) and reduced the maximum oxidation rate by 46 +/- 5%. Ascorbic acid, at 9 microM, increased the lag time by 179 +/- 29 min (6-fold compared to controls) but did not affect the maximum oxidation rate. The increase in lag time induced by MyB was enhanced in the presence of ascorbic acid. Their effects were additive, except when both were present at the highest concentration tested, when a significant potentiation, giving an increase in lag time of approximately 2 hr more than the sum of separate effects, occurred. Concentration-time curves for MyB in the absence and presence of ascorbic acid showed that the vitamin protected MyB against deterioration during incubation, and indicated that the net consumption of MyB in the oxidation process was reduced. No differences were observed when ordinary ascorbic acid and Ester-C, a commercial vitamin C product, were compared. In conclusion, MyB and ascorbic acid seem to interact in a way that further improves the antioxidant status of the LDL particle relative to each substance separately.