We describe the assay of the human cytochrome P450 2D6 in a set of 30 genotyped liver samples using the 'absolute quantification' (AQUA) technique. We found approximately 30 fmol CYP2D6 per microg of microsomal protein, with the values spanning from 0 to nearly 80 fmol/microg. This is greater by a factor of 5-10 from the compared to the currently accepted value, which was around 5 fmol/microg. Our results thus suggest that the amount of cytochrome P450 (CYP) enzymes in liver have to be reassessed. We used quantitative Western blotting, calibration standards and activity assays, to validate the results. Our results show, that using the AQUA technique a true assay of CYP2D6 in human liver was possible.