Sulfotransferases are phase II drug-metabolizing enzymes. While the induction of sulfotransferases by hormones and endogenous molecules is relatively well known, induction by xenobiotics is not well studied. Isoflavones are naturally occurring phyto-oestrogens, mainly existing in soy food products. They have been described as health-promoting, disease-preventing dietary supplements and as agents with cancer-preventive activities. Recently, isoflavones have been reported to interact with nuclear receptors, including those that are known to mediate the induction of drug-metabolizing enzymes. In the present investigation, the isoflavone genistein was shown to be a xenobiotic inducer of human sulfotransferases in transformed human liver cells (HepG2) and colon carcinoma cells (Caco-2). Enzymatic activity assay, Western blot, and real-time reverse transcription-polymerase chain reaction (RT-PCR) results demonstrated that genistein significantly induced protein and mRNA expression of human simple phenol sulfotransferase (hSULT1A1) and human dehydroepiandrosterone sulfotransferase (hSULT2A1) in HepG2 and Caco-2 cells. The induction was time-dependent and dose-dependent. Western blot results agreed well with real-time RT-PCR results, suggesting that induction occurred at the gene transcription level. This isoflavone is the first nutritionally related phyto-oestrogen shown to induce human sulfotransferases in HepG2 and Caco-2 cells.