To analyze the inhibitory effects of isothiocyanates on the growth of human cancer cells, the effects of these compounds on the cell cycle progression of HeLa cells were studied by flow cytometry. The accumulation of cells at G2/M phase was observed at 16 h after treatment with 10 microM allyl isothiocyanate, 2.5 microM benzyl or phenethyl isothiocyanate, at which concentrations the cell growth was inhibited to 41-79% of control. These results suggest that isothiocyanates delay the cell cycle progression of HeLa cells, leading to inhibition of cell growth.