The interior of living cells is occupied by macromolecules such as proteins, which occur at a high volume fraction on the order of 30% in the aqueous solution of the cellular fluid. The issue of macromolecular crowding is therefore of primordial importance for the function of living cells [R.J. Ellis, Curr. Opin. Struct. Biol. 2001, 11, 114]. To model the situation of crowding in vitro, we propose to investigate the diffusion of protein(BSA)-protein(IgG)-water(D2O) ternary mixtures, where the proteins BSA (bovine serum albumin) and IgG (immunoglobulin) are important constituents of blood. Using our previous large QENS data sets of BSA-water and IgG-water binary mixtures and consistent modeling, we expect to be able to separate the contributions of the two proteins to the QENS spectra in the ternary mixture.