1. The aim was to compare the metabolic activity of human CYP3A4 expressed in bacteria (E. coli), yeast (S. cerevisiae) and human lymphoblastoid cells (hBl), with the native CYP3A4 activity observed in a panel of human livers. 2. Three CYP3A4 substrates were selected for study: dextromethorphan (DEM), midazolam (MDZ) and diazepam (DZ). The substrate metabolism in each of the four systems was characterized by deriving the kinetic parameters Km or S50, Vmax and intrinsic clearance (CLint) or maximum clearance (CLmax) from the kinetic profiles; the latter differing by 100-fold across the three substrates. 3. The Km or S50 for the formation of metabolites 3-methoxymorphinan (MEM), 1′-hydroxymidazolam (1′-OH MDZ) and 3-hydroxydiazepam (3HDZ) compared well in all systems. For CYP3A4-mediated metabolism of DEM, MDZ and DZ, the Vmax for hB1 microsomes were generally 2-9-fold higher than the respective yeast and human liver microsomes and E. coli membrane preparations, resulting in greater CLint or CLmax. In the case of 3HDZ formation, non-linear kinetics were observed for E. coli, hBl microsomes and human liver microsomes, whereas the kinetics observed for S. cerevisiae were linear. 4. The use of native human liver microsomes for drug metabolic studies will always be preferable. However, owing to the limited availability of human tissues, we find it is reasonable to use any of the recombinant systems described herein, since all three recombinant systems gave good predictions of the native human liver enzyme activities.