The partitioning of IgG in aqueous two-phase systems (ATPSs) of PEG and Dextran was studied systematically using a statistical experimental design. Aim was to improve the separation of hybridoma cells and their IgG product, by identifying the key variables governing IgG partitioning, and by comparing the IgG partitioning data with the hybridoma cell partitioning data obtained in previous work. The influence of five factors, i.e. the poly(ethylene glycol) molecular weight (PEG Mw), dextran molecular weight (Dx Mw), tie-line length (TLL), pH and potassium phosphate fraction (KPi/(KPi+KCl)), on IgG partitioning was characterized using a full-factorial experimental design. In all of the ATPS's the IgG partitioned predominantly into the lower phase. The partition coefficient varied between 0.78 (Variable settings: PEG Mw = 6000, Dx Mw = 500000, TLL = 0.10 g g-1, KPi/(KPi+KCl) = 1.0 and pH = 7.4) and 0.0002 (Variable settings: PEG Mw = 35000, Dx Mw = 40000, TLL = 0.20 g g-1 KPi/(KPi+KCl) = 1.0 and pH 6.6). The tie-line length, the dextran molecular weight and the PEG molecular weight had the most pronounced effect on IgG partitioning. Matching the partitioning data of the IgG product with previously obtained data of the hybridoma cell partitioning, showed that within the experimental design no ATPS could be found giving a good separation of the hybridoma cells and their IgG product. There are, however, ATPS's available in which the cells partition to, and grow in the lower dextran-rich phase. To achieve a good separation of the hybridoma cells and their IgG product in these ATPSs, the IgG product has to be specifically extracted into the PEG-rich top phase. For this purpose the use affinity ligands coupled to PEG may offer a solution. Therefore, a number of commercially available dye-resins was screened for their ability to bind the BIF6A7 IgG antibody. The mimetic green 1 A6XL dye-resin was found to bind BIF6A7 IgG. The dye-ligand coupled to PEG was used to manipulate the IgG partitioning in an ATPS. In the presence of the PEG-ligand, the IgG partitioned almost completely to the top phase. The IgG-partition coefficient increased three orders of magnitude, resulting in a 25-fold higher IgG concentration in the top phase than in the bottom phase.