Molecular mechanisms of the bile acid active transport system in the ileal enterocytes remain unknown. We examined whether bile acids affect human enterocyte gene expression of intestinal bile acid-binding protein (I-BABP), a component of this transport system. Differentiated Caco-2 cells were incubated in the presence of human bile, bile acids or other lipids. The level of I-BABP expression was evaluated by Northern and Western blot analyses. A 24 h incubation of Caco-2 cells in a medium containing either bile or bile acids resulted in a remarkable 7.5-fold increase in the I-BABP mRNA level over the control level. Neither cholesterol, palmitic acid, phosphatidylcholine nor cholestyramine treated bile showed any difference in I-BABP mRNA expression from the control. Bile acid treatment increased the level of I-BABP mRNA in Caco-2 cells in a time- and dose-dependent manner. Western blot analysis showed that this induction led to increase in cytosolic I-BABP. Chenodeoxycholic acid and deoxycholic acid showed greater induction effects than other hydrophilic bile acids, including their own glycine conjugates. Pretreatment by actinomycin D or cycloheximide completely inhibited the up-regulation of I-BABP expression by bile acid. Bile acids, especially lipophilic bile acids, increase the I-BABP expression in Caco-2-cells, suggesting that luminal bile acids play an important role in regulating the I-BABP gene expression.