We investigated the absorption and metabolism of quercetin, a typical flavonoid, in Caco-2 cells, which are obtained from a human colon adenocarcinoma cell line and have been used as a good model for the small intestinal epithelial cells. After incubation of Caco-2 cells with quercetin, cell homogenates were treated with sulfatase type H-5, which releases quercetin from its glucuronides and sulfates. HPLC analysis of the homogenates with an electrochemical detector revealed absorption of quercetin by the cells. The amount of quercetin in the cell homogenates treated with sulfatase type H-5 was much higher than that in the cell homogenates before treatment with the enzyme. These results indicate that quercetin was conjugated easily in Caco-2 cells and suggest that similar conjugation of flavonoids probably occurs in the intestinal epithelial cells.