The solution thermodynamics and interactions of a reversibly self-associating IgG1 monoclonal antibody have been investigated as a function of cosolute type (NaCl, NaSCN, arginine-HCl) and cosolute concentration over a wide range of protein concentrations (1-235 mg/mL) using static light scattering. The nonideality of mAb solutions is analyzed within the simplifying framework of a two-component system to obtain the dependencies of the excess chemical potential of the mAb on protein and cosolute concentrations. Using hard spheres as a model of mAbs in the absence of intermolecular interactions, the mean interparticle distances can be estimated as a function of antibody concentration. Analysis of MAb1 excess chemical potential and mean intermolecular distance results in a potential function representing the sum of protein-protein interactions and their contributions to solution nonideality. This approach facilitates evaluation of the relative contributions of attractive/repulsive intermolecular interactions and excluded volume effects, as well as the effects of cosolutes on protein multiparticle interactions in crowded conditions. Underlying the dominant effect of volume exclusion at high protein concentrations, attractive interactions were found to be amplified with decreasing intermolecular distances by the MAb1 many-body correlations. Comparison of the cosolute concentration dependence of the protein chemical potential, d mu(ex)(2)/dC(3), across the mAb concentrations demonstrates that MAb1 self-association is reduced with increasing ionic strength and in a series based on cosolute identity; Arg-Cl > NaSCN > NaCl. The effectiveness of arginine-HCl and NaSCN in modulating MAb1 excess chemical potential in concentrated solutions is ascribed to the cosolute's ability to mitigate both electrostatic as well as weaker hydrophobic attractive interactions between MAb1 molecules. This investigation presents the first direct analysis of cosolute specific effects on protein-protein interactions at high concentrations, and provides a novel approach for characterizing the many-body effects that contribute to solution nonideality.