Protein diffusivity in rabbit lenses was observed in vivo with laser light-scattering spectroscopy. Levels of laser light irradiation, estimated to correspond to a level less than that used in routine ophthalmoscopy, were adequate for observation. The in vivo measurements agreed with measurements on the same lens after excision from the animal. Since changes in protein diffusivity reflecting even very small degrees of protein aggregation can be observed by this method, it should make possible quantitative studies of incipient lens opacification.