We report a detailed comparison of two commonly used stable, amplifiable mammalian expression systems (Chinese Hamster Ovary cells/dihydrofolate reductase and Mouse NSO myeloma/glutamine synthetase) used to express a humanized IgG1 monoclonal antibody. We compare copy number and steady state mRNA levels of both the selectable marker and heavy chain of the antibody throughout the selection and amplification process. In both cell lines, copy number and steady state levels of heavy chain and selectable marker increased during selection and were further increased during amplification. As expected, an increase in steady state mRNA levels of heavy chain correlated with an increase in expression of antibody whilst an increase in the steady state levels of mRNA of the selectable marker correlated with increased resistance to the selective agent. In NSO and CHO cells producing equivalent amounts of antibody, the copy number of the antibody genes and selectable marker was significantly higher in the CHO cells than in the NSO cells. However, the steady state mRNA levels of the heavy chain of the antibody were virtually identical. Rates of protein secretion in the two cell lines were also compared and found to be very similar. When the antibody purified from both systems was compared in a number of functional assays they behaved identically.