Controlling the viscosity of concentrated protein solutions - usually reducing - is an open challenge, with major recent relevance in protein formulations for biopharmaceutical, medical, food, and other applications. The addition of viscosity-reducing additives generally not only changes the viscosity of the protein solutions but also the actual secondary/tertiary structure of the proteins, which is usually highly undesirable, and can be even toxic in systems, such as for biopharmaceutical applications. Therefore, it is of major importance to be able to establish control over the combination of viscosity-affecting additives and adequate protein stability, usually at high protein concentrations. Here, we demonstrate the control and manipulation of the viscosity profile of a selected protein solution (monoclonal antibody of immunoglobulin gamma type IgG) of direct biopharmaceutical relevance, by identifying elementary viscosity contributions via selected additives that target different protein-protein interactions. Specifically, a combined study of viscosity control and protein aggregation is performed with viscosity characterized by microfluidic measurements and protein aggregation by size-exclusion chromatography, where aggregation data is further supplemented with conformational stability measurements via thermal and chemical protein denaturation. A dissection of contributions to total viscositysteric, electrostatic, hydrophobic, van der Waals - is performed. A novel mechanism of the impact of electrostatic interactions on the viscosity of IgG solutions is proposed based on interacting charged protein patches subjected to orientational alignment under flow birefringence. More generally, we show a control over the interplay of viscosity, potency and stability in a distinct protein system, as a general contribution to understanding the viscosity in different colloidal, biological, and soft materials. (C) 2020 Elsevier Inc. All rights reserved.