Ginseng extract has been reported to decrease the incidence of 7,12-dimethylbenz[a]anthracene (DMBA)-initiated tumorigenesis in mice. A potential mechanism for this effect by ginseng is inhibition of DMBA-bioactivating cytochrome P450 (P450) enzymes. In the present in vitro study, we examined the effect of a standardized Panax ginseng (or Asian ginseng) extract (G115), a standardized Panax quinquefolius (or North American ginseng) extract (NAGE), and individual ginsenosides (Rb1, Rb2, Rc, Rd, Re, Rf, and Rg1) on CYP1 catalytic activities, as assessed by 7-ethoxyresorufin O-dealkylation. G115 and NAGE decreased human recombinant CYP1A1, CYP1A2, and CYP1B1 activities in a concentration-dependent manner. Except for the competitive inhibition of CYP1A1 by G115, the mode of inhibition was the mixed-type in the other cases. A striking finding was that NAGE was 45-fold more potent than G115 in inhibiting CYP1A2. Compared with G115, NAGE also preferentially inhibited 7-ethoxyresorufin O-dealkylation activity in human liver microsomes. Rb1, Rb2, Rc, Rd, Re, Rf, and Rg1, either individually or as a mixture and at the levels reflecting those found in an inhibitory concentration (100 microg/ml) of NAGE or G115, did not influence CYP1 activities. However, at a higher ginsenoside concentration (50 microg/ml), Rb1, Rb2, Rc, Rd, and Rf inhibited these activities. Overall, our in vitro findings indicate that standardized NAGE and G115 extracts, which were not treated with calf serum or subjected to acid hydrolysis, inhibited CYP1 catalytic activity in an enzyme-selective and extract-specific manner, but the effects were not due to Rb1, Rb2, Rc, Rd, Re, Rf, or Rg1.