A process for the effective extraction and fractionation of phlorotannins from 
Fucus vesiculosus with high antioxidant potentials was investigated. The 
antioxidant activity of F. vesiculosus extract/fractions was assessed by 
2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging, reducing power, and 
ferrous ion-chelating assays. Among the crude extract and different polarity 
fractions, the phlorotannin-enriched ethyl acetate fraction possessed the 
highest DPPH scavenging activity and reducing power. This fraction was further 
fractionated by Sephadex LH-20 column chromatography or ultrafiltration. The 
antioxidant properties were evaluated by both the above chemical antioxidant 
tests and a mononuclear cell-based bioassay. Sephadex subfractions LH-2 and LH-3 
with high total phlorotannin content exhibited strong DPPH quenching activity, 
comparable to those of ascorbic acid and butylated hydroxytoluene and 
significantly higher than that of α-tocopherol. Polyphenols in F. vesiculosus 
were found to consist mainly of high molecular weight phlorotannin polymers. 
There were no clear relationships between the degree of polymerization, 
molecular size, and antioxidant activity. All the subfractions separated by 
Sephadex LH-20 column chromatography and ultrafiltration showed a high ability 
to scavenge reactive oxygen species generated by mononuclear cells. Further 
characterization of the phlorotannin compounds was performed on six Sephadex 
subfractions. Several phlorotannin oligomers were tentatively identified on the 
basis of HPLC-ESI-MS(n) analyses.