The current work investigates the extraction of a monoclonal antibody immunoglobulin G1 from a Chinese hamster ovary cell supernatant. Solubility of biologic substances was figured out as the most limiting factor during aqueous two-phase extraction. Therefore, it was majorly considered for system choice and for the determination of an operating window to prevent product loss due to precipitation. The solubility of immunoglobulin G1 was screened for different solutions of phase forming components. Best solubility was observed for a polyethylene glycol 2000 - phosphate aqueous two-phase system at pH six. The influence of additional sodium chloride and cell supernatant loading on the purification was investigated. Most promising extraction conditions were determined to either include no or a high amount of sodium chloride. A decrease of product phase (immunoglobulin G1-rich phase) volume further improves the purification resulting purification factors of up to 3.1 with an immunoglobulin G1 yield of higher than 90% within a single extraction step. (C) 2014 Elsevier B.V. All rights reserved.