HepaRG is a highly differentiated cell line that displays several hepatocyte-like functions, including drug-metabolizing enzymes. In this study, the HepaRG cells were characterized and evaluated as an in vitro model to predict cytochrome P450 (P450) enzyme induction of drugs in humans. Exposure of HepaRG cells to prototypical inducers resulted in induction of CYP1A1, CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, and CYP3A4 mRNA, as well as phenacetin O-dealkylase, bupropion hydroxylase, diclofenac 4'-hydroxylase, and midazolam 1'-hydroxylase activities. The observed induction is consistent with the previously reported expression of the nuclear receptors pregnane X receptor, constitutive androstane receptor, and aryl hydrocarbon receptor, which are necessary for a P450 induction response. To avoid problems with toxicity and solubility, the induction potency of test compounds was evaluated by calculating the concentrations leading to a 2-fold increase of baseline mRNA or enzyme activity levels (F(2) values) instead of EC(50) values from full dose-response curves. For CYP3A4 mRNA, the obtained F(2) values were related to the in vivo exposure [area under the plasma concentration versus time curve (AUC)] of the inducer (AUC/F(2)). This score was then correlated with the decrease in AUC for a CYP3A probe drug, administered before and after treatment with the inducing agent. By using this method an excellent correlation (R(2) = 0.863) was obtained, which implies that the degree of CYP3A induction in vivo can be predicted from CYP3A4 mRNA induction in HepaRG cells. The present study shows that HepaRG cells are a valuable model to be used for prediction of induction of drug-metabolizing P450 enzymes in vivo in humans.