Ascorbigen (ABG) is formed by indole-3-carbinole and ascorbic acid in brassica vegetables. In the present study, ABG has been systematically analyzed for its free radical scavenging and antioxidant capacity. To directly measure the free scavenging activity of ABG and ascorbic acid (used as a positive control), electron spin resonance and spin trapping experiments were performed. Ascorbic acid exhibited a potent free radical scavenging activity, whereas ABG did not scavenge superoxide anion free radicals and showed only little scavenging activity toward 2,2-diphenyl-1-picrylhydrazyl free radicals. Similar data were obtained for the ferric reducing ability of plasma and trolox equivalent antioxidant capacity assays. In cultured human keratinocytes, ABG counteracted tert-buthylhydroperoxide-induced cytotoxicity, whereas ascorbic acid did not exhibit any protective activity. Furthermore, in ABG-treated human keratinocytes, a decrease in tert-buthylhydroperoxide-induced lipid peroxidation was detected, whereas an ascorbic acid pretreatment did not result in the prevention of lipid peroxidation. These data indicate that ascorbic acid seems to be a more potent free radical scavenger than ABG in vitro, while ABG prevented tert-buthylhydroperoxide cytotoxicity more effectively as compared to ascorbic acid in cultured cells.