Improvements of the upstream process for the production of monoclonal antibodies (mAb) has resulted in mammalian cell cultures with significantly increased cell densities. These high cell density (HCD) broths pose severe challenges for established downstream unit operations due to the increased biomass load. To meet these challenges, aqueous two-phase extraction was examined as a potential method for the clarification, as well as first stage purification of a Chinese hamster ovary expressed mAb with cell densities up to 70 center dot 10(6)cells mL(-1). A clarification with 99 % yield of mAb and 68 % DNA removal was accomplished, which contributes to enable a foundation to the industrial application of aqueous two-phase extraction for manufacturing of mAb from HCD cultivations.