The viscosities, eta, of ternary mixtures of water (1) + dimethyl sulfoxide (2) + lysozyme (3) were measured, and the intrinsic viscosities of lysozyme were determined by extrapolating the reduced viscosities to infinitely dilute lysozyme concentration at 298.15 K. A binary mixture of water + dimethyl sulfoxide had a maximum deviation of viscosity at approximately 0.33 mol fraction of dimethyl sulfoxide (x(2)). Lysozyme unfolded and aggregated with an accompanying increase in the intrinsic viscosity of lysozyme at x(2) = 0.3-0.4. These results indicated that the strong interaction between water and dimethyl sulfoxide might influence the conformation of lysozyme by interfering interactions between water and lysozyme.