We report on the size, shape, structure, and interactions of lysozyme in the ternary system lysozyme/DMSO/water at low protein concentrations. Three structural regimes have been identified, which we term the "folded" (0 < φ(DMSO) < 0.7), "unfolded" (0.7 ≤ φ(DMSO) < 0.9), and "partially collapsed" (0.9 ≤ φ(DMSO) < 1.0) regime. Lysozyme resides in a folded conformation with an average radius of gyration of 1.3 ± 0.1 nm for φ(DMSO) < 0.7 and unfolds (average R(g) of 2.4 ± 0.1 nm) above φ(DMSO) > 0.7. This drastic change in the protein's size coincides with a loss of the characteristic tertiary structure. It is preceded by a compaction of the local environment of the tryptophan residues and accompanied by a large increase in the protein's overall flexibility. In terms of secondary structure, there is a gradual loss of α-helix and concomitant increase of β-sheet structural elements toward φ(DMSO) = 0.7, while an increase in φ(DMSO) at even higher DMSO volume fractions reduces the presence of both α-helix and β-sheet secondary structural elements. Protein-protein interactions remain overall repulsive for all values of φ(DMSO). An attempt is made to relate these structural changes to the three most important physical mechanisms that underlie them: the DMSO/water microstructure is strongly dependent on the DMSO volume fraction, DMSO acts as a strong H-bond acceptor, and DMSO is a bad solvent for the protein backbone and a number of relatively polar side groups, but a good solvent for relatively apolar side groups, such as tryptophan.