1. Induction of endogenous cytochrome P4501A1 (CYP1A1) by benzimidazole derivatives has been investigated in the human hepatoma cell line HepG2. 2. By Northern and Western blot analysis, omeprazole has been shown to be a more potent inducer of CYP1A1 than both lansoprazole and E3810, whereas pantoprazole did not induce CYP1A1. Similar results were obtained for the CYP1A1 enzyme-specific deethylation of 7-ethoxyresorufin. 3. The induction of CYP1A1 in the permanent cell line HepG2 corresponds to results observed in human hepatocytes in primary culture. 4. The results provide experimental evidence that HepG2 cells can be used as an appropriate tool to examine inducing effects of drugs on the expression of CYP1A1.