The present study represents a comparison of three approaches to transform recombinant cytochrome P-450 (rCYP) enzyme kinetic data to human liver activity using mirtazapine (MIR) biotransformation as a model. MIR metabolite rCYP formation rates were corrected using I) relative activity factors (RAFs) determined on site, II) RAFs based on activity data provided by the rCYP manufacturer, and III) immunologically determined human liver abundance of CYP isoforms reported in the literature. For 2.5, 25, and 250 microM MIR, predictions of 1) the relative contribution of CYP isoforms to a particular reaction, 2) absolute metabolite formation rates, 3) the relative contribution of each pathway to net MIR biotransformation, and 4) the relative contribution of CYP isoforms to net MIR biotransformation were generated, and the results were compared with data obtained with human liver microsomes (HLM). We found that RAFs determined on site most accurately predict the results observed in HLM. Estimations based on liver abundance systematically underestimated CYP1A2 and overestimated CYP3A and CYP2C9 contributions to MIR metabolism and, therefore, seem less suitable to predict CYP isoform involvement in a particular reaction. Normalized RAFs calculated from the manufacturer activity data fell within the range of RAFs determined on site and lead to similar results for CYP isoform contribution to metabolic reactions and to net MIR biotransformation. Considering the time and resource-intensive step of RAF determination, manufacturer RAFs are an alternative to RAFs determined on site for the transformation of rCYP enzyme kinetic data; both of them provide more accurate estimations than immunologically determined human liver CYP isoform content.