Dynamic light scattering (DLS) is a useful technique for analyzing the size, shape, and other structural characteristics of protein molecules in solution. The effects of various environmental conditions on the structure and activity of Aspergillus oryzae beta-galactosidase were studied. DLS was used to determine protein particle size under various salt, pH, and temperature conditions. Changes in the activity and stability of this enzyme caused by different environmental conditions were found to correlate well with the size changes of the protein particles. This change in protein size can be attributed to protein unfolding and aggregation under extreme conditions. The presence of the enzyme substrate, lactose, in the protein solution greatly enhanced enzyme stability by inhibiting aggregation.