Several studies have implicated the potent antioxidant properties of lycopene. However, most of the studies used only the (all-E)-isomer. (Z)-Isomers of lycopene were found in substantial amounts in processed foods and in human tissues. In the present study, we investigated in vitro the antioxidant activity of (5Z)-, (9Z)-, and (13Z)-lycopene compared to the (all-E)-isomer. Additionally, prolycopene, the (7Z,9Z,7 0 Z,9 0 Z)-isomer found in tangerine tomatoes, was analyzed. No significant differences were found between the isomers in ferric reducing antioxidant power assay and in bleaching the radical cation of 2,2 0-azinobis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), both based on ET mechanisms. In contrast, scavenging activity against peroxyl radicals generated by thermal degradation of 2,2 0-azobis(2-amidinopropane) (AAPH) was higher in the (Z)-isomers. (5Z)-Lycopene was most antioxidant in scavenging lipid peroxyl radicals, evaluated by analyzing the inhibition of MbFe III lipid peroxidation of linoleic acid in mildly acidic conditions (pH 5.8) in a micellar environment, modeling a possible antioxidant action in the gastric compartment. ' INTRODUCTION Lycopene, β-carotene and lutein/zeaxanthin are the most abundant carotenoids detected in the blood of people from European countries. 1 The polyisoprenoid structure of carote-noids makes them susceptible to enzymatic 2 or chemical oxidative cleavage 3 and isomerization from (E)-to (Z)-forms. Both (Z)-isomers and cleavage products of carotenoids (apocarotenoids) can be regarded as potentially bioactive metabolites. Lycopene is an acyclic carotenoid having 11 conjugated double bonds in (all-E)-isomer and various (Z)-configurations (see Figure 1), as originally suggested by Zechmeister in 1938. 4 Lycopene in fruits and vegetables occurs mostly (80À97%) in the (all-E)-configuration. 5 Food processing and preparation of lycopene-containing meals increase the proportion of lycopene (Z)-isomers. The proportion of (Z)-isomers varied from 4% in preserved tomato paste up to 65% in long-term cooked spaghetti sauce prepared from canned tomatoes under household conditions (45À60 min at 85À110 °C). Additionally, the order of increasing proportion of lycopene isomers in this food sample was observed as (13Zþ15Z) , (9Z) < (5Z) < (all-E). 6 Lycopene (Z)-isomers also play a quantitatively important role in human plasma and tissues 6,7 where levels of (Z)-isomers range up to more than 50% of total lycopene. The most abundant (Z)-isomers of lycopene in human blood plasma are (5Z)-, (9Z)-and (13Z)-Lycopene, 6À8