The generalized Stokes-Einstein equation, which predicts a linear dependence of the diffusion coefficient upon concentration for dilute solutions, has been investigated for highly concentrated solutions by photon correlation spectroscopy. The globular protein ovalbumin was found to exhibit a well-localized peak in its mutual diffusion coefficient at a concentration of ≈ 100 mg/ml, superimposed on a general linear trend to smaller values with incresing concentration. The appearance of the peak can be explained in a semi-quantitative manner by the Debye-Huckel theory for macro-ionic interactions.