Chemoprevention using isothiocyanates is partly the result of the induction ofphase II enzymes for carcinogen detoxification from healthy cells. However,phase II enzyme activity can impair cancer therapeutic agents. The objective ofthis study was to assess phase II enzyme induction and cytotoxicity of phenethylisothiocyanate (PEITC) and glutathione conjugate with PEITC (GsPEITC) usingMCF-7 and MDA-MB-231 breast cancer cells. Changes of phase II enzymes,glutathione-S-transferase (GST) and NAD(P)H quinone reductase (QR), weremeasured by colorimetric procedures. Cell viability was determined using theMTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-di-phenyltetrazolium- bromide) assay.From current results, 24hr exposure to 50 μM) of PEITC or GsPEITC increased phase IIenzyme activity by a maximum of 500-700-percent. To conclude, cytotoxic dosesof PEITC or GsPEITC possess phase II enzyme inducing activity in MCF-7 andMDA-MB-231 breast cancer cells.