OBJECTIVE: To observe the effect of Indole-3-carbinol (I3C), a naturally occurring component of cruciferous vegetables, on cell proliferation of a colon cancer cell line. METHODS: Cell proliferation was measured using three different methods; 3H-thymidine incorporation, cell count and colourimetric assay. RESULTS: Each method of measurement revealed that I3C significantly reduced cell proliferation at concentrations of > 0.1 mM. CONCLUSION: The present study demonstrates for the first time the capacity of indole-3-carbinol to inhibit cell proliferation of a colon cancer cell line.