Transient gene expression (TGE) is a well-established enabling technology for rapid generation of recombinant proteins, with Human Embryonic Kidney (HEK) and Chinese Hamster Ovary (CHO) cell lines and polyethyleneimine (PEI) as the transfection reagent being its most popular components. However, despite considerable progress made in the field, volumetric titers can still be a limiting factor causing the manipulation of increasing quantities of culture media and DNA. Here, we report a systematic analysis of TGE conditions and their influence on yields and protein quality. Guided by Design of Experiments (DoE), we conclude that TGE yields with one test antibody can be maximized by a parallel increase of cell density - 2.4 to 3.0 × 10(6)cells/mL - and PEI concentration - 24 to 30 mg/L - while maintaining a 1:1 ratio of heavy chain and light chain encoding plasmids. Interestingly, we also show that in these conditions, DNA concentration can be maintained in the 1mg/L range, thereby limiting the need for large DNA preparations. Our optimized settings for PEI-mediated TGE in HEK and CHO cells evaluated on several proteins are generally applicable to recombinant antibodies and proteins.