Static and dynamic laser light scattering are discussed as tools for studying protein-protein interactions, protein oligomerization, and aggregation. Dynamic light scattering (DLS) is very well suited for detection of small quantities of aggregates in protein samples because DLS can easily analyze samples with broad distribution of sizes. No fractionation is used and sample is analyzed in batch mode without any chromatographic separation. DLS detector can also be used as an �on-line� detector coupled with a fractionation step.

Static light scattering (SLS) is best utilized for measurement of molar masses as an �on-line� detector coupled with size exclusion chromatography (SEC), refractive index (RI) and ultraviolet (UV) detection. Since static light scattering provides only the weight-average molar mass, Mw, of the species in solution, the SEC separation plays an integral role in the overall analysis, albeit, the elution from SEC does not need to correlate with the molar masses of the species being studied. SEC/LS allows determination of molar mass of unmodified proteins with a precision of �5% in a single experiment that uses <100 �g of protein; for DLS �on-line� measurement, ~400�g is needed. Monitoring the elution from SEC by three detectors, UV, LS and RI, provides an excellent tool for detection of sample heterogeneity. Potential loss of protein on the SEC column, sample dilution, and restriction on elution solvent are major limitations of SEC/LS analysis. SEC/LS is suitable for analysis of glycoproteins, proteins modified by polyethylyne glycol as well as membrane proteins solubilized in non-ionic detergents.