The ability of indole-3-carbinol (indole-3-methanol) to trap a metastable synthetic-free radical is presented. Indole-3-carbinol is capable of acting as a scavenger of free radicals in an in vitro system. The presence of indole-3-carbinol determines the disappearance of the free radicals, the reaction being time- and concentration-dependent. The scavenging activity of different indoles is compared. Indole-3-carbinol and indole-3-acetic acid are both able to scavenge free radicals, but indole-3-carbinol is more effective. Other indoles such as indole-3-aldehyde and indole-3-carboxylic acid do not show the ability to trap free radicals. Indole-3-aldehyde appears as a product of indole-3-carbinol reaction with free radicals. The formation of an adduct between the free radical generated in vitro and indole-3-carbinol has also been detected. Stability of indole-3-carbinol in buffered media at different pH values and formation of 3,3'-diindolylmethane from indole-3-carbinol is also studied. The scavenging activity of indole-3-carbinol and its implications on the anti-carcinogenesis process is discussed.