Combined antioxidative effects of lettuce extract and alpha-tocopherol, quercetin or ascorbic acid (AA) were investigated for peroxidation of L-alpha-phosphatidylcholine liposomes with oxidation initiated by lipophilic or by hydrophilic azo-initiators. Lettuce extract had a clear antioxidative effect as evidenced by a lag phase for formation of conjugated dienes, and alpha-tocopherol and especially quercetin acted synergistically in prolongation of the lag phase both following initiation in the lipid phase and in the aqueous phase. Combination of AA with lettuce extract showed in contrast a lag phase that was similar to that observed for AA alone. Storage of lettuce extract for 24 hat refrigerator or room temperatures resulted in a Decreasing lag phase with increasing storage time for both storage temperatures, an effect not counteracted by addition of quercetin or alpha-tocopherol. Heating of lettuce extract for 10 min at 80 degrees C did not affect the lag phase and heating of lettuce extract resulted in an increasing synergism for added quercetin and alpha-tocopherol indicating that thermal inhibition of polyphenoloxidase (PPO) increases antioxidant potential and interaction. (C) 2008 Elsevier Ltd. All rights reserved.