Lignin is one of the major components of dietary fiber. It is a complex hydrophobic molecule that typically occurs in cell walls with heteroxylans. Our experimental data show that lignin is a free radical scavenger. When the NADH-phenazine methosulfate-nitro blue tetrazolium free radical-producing system is used, an alkali-lignin concentration of 46.29 micrograms/ml that causes 50% inhibition of uric acid production by xanthine oxidase (IC50) is a scavenger of superoxide anion radicals. Spectrophotometric assay has shown that alkali-lignin with an IC50 of 59.08 micrograms/ml inhibits the activity of xanthine oxidase, one of the enzymes related to the production of superoxide anion radicals, and presents a mixed-type noncompetitive inhibition pattern. Using the deoxyribose method, we have found that alkali-lignin is a hydroxyl radical scavenger with an IC50 of 250 micrograms/ml, and using the thiobarbituric acid method, we can see that alkali-lignin inhibits nonenzymatic and enzymatic lipid peroxidation with an IC50 of 72 and 100 micrograms/ml, respectively. Alkali-lignin also hinders the activity of glucose-6-phosphate dehydrogenase, another enzyme related to the generation of superoxide anion radicals, with an IC50 of 123.6 micrograms/ml, and obstructs the growth and viability of cancer (HeLa) cells in a dose-dependent manner. Our experimental results suggest another mechanism whereby the free radical-scavenging activity of lignin in dietary fiber may be involved in the fiber-colon cancer interaction. We also suggest that the ability of dietary fiber to protect against colon cancer may be partly determined by the amount of lignin in dietary fiber as well as the free radical-scavenging ability of lignin.