The sedimentation equilibrium of solutions of immunoglobulin G in saline buffer, over a concentration range up to 125 g/L, was measured and analyzed in the context of a model that takes into account the possibility of attractive intermolecular interaction leading to the reversible formation of oligomeric species and repulsive intermolecular interaction leading to nonideal solution behavior. Additionally, previously published data on the concentration dependence of the osmotic pressure of immunoglobulin G under similar conditions, over a concentration range up to 400 g/L, were analyzed in the context of a newly developed thermodynamic formalism describing the osmotic pressure of a solution containing multiple nondiffusible solute species at an arbitrary concentration. Both sets of data are quantitatively accounted for by a model in which IgG self-associates at very high concentration to form (predominantly) trimers under the conditions of these experiments.