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ABSTRACT Assessment of the oral use of indole-3-carbinol (I3C) as a chemoprotective compound has not sufficiently considered the chemical instability of I3C. This review addresses the question of whether I3C is directly active in its own right or only serves as a precursor, with all of the biological responses coming from reaction products arising in culture media and in the presence of stomach acid. Because of the rapid conversion of I3C into its dimer. diindolylmethane (DIM), and trimers very little circulating I3C is present following oral use to effect a biological response. Reports of toxicity associated with oral use of I3C relate to unfavorable enzyme induction, which can be attributed to non-DIM reaction products. Because DIM provides a predictable, safer response than the mélange of compounds derived from I3C DIM should be regarded as the chemoprotective compound of choice.