The level of spectrally active P450 was determined from the Fe²⁺ versus Fe²⁺-CO difference spectrum of total cellular protein isolated from the various cell lines. The following enzyme activities were assayed in cultured cells: bufuralol-hydroxylase (CYP2D6) and testosterone 6β-hydroxylase (CYP3A4). The level of hOR in the various recombinant cell lines was between 190 and 225 nmol of cytochrome c reduced/min/mg of total cellular protein, except the cell line that expressed CYP3A4 alone and the parental cell line for which this value was between 7 and 12 nmol of cytochromec reduced/min/mg of total cellular protein. Testosterone 6β-hydroxylase and bufuralol 1′-hydroxylase activities were reported to be 5000 and 70 nmol/min/mg of protein, respectively, in human liver microsomes (reviewed in Friedberg et al., 1999. Data for total cellular protein isolated from hepatocytes were not available.