Aggregation is common in protein drug manufacture, and while the effects of protein particulates are under investigation, many techniques applicable for their characterization have been recently developed. Among the methods available to characterize and quantify protein aggregates, none is applicable over the full size range and different methods often give conflicting results. The studies presented here compare two such methods: dynamic light scattering (DLS) and resonant mass measurement (RMM). The performance of each method was first characterized using polystyrene particle size standards (20, 60, 100, 200, 400, and 1,000 nm) over a range of concentrations. Standard particles were measured both singly and in binary mixtures containing 20 nm particles at a fixed concentration (10(14) particles/mL) and various concentrations of one of the other particle sizes (i.e., 60, 100, 200, 400, or 1,000 nm). DLS and RMM were then used to detect unknown aggregate content in stressed samples of IgG. Both instruments were shown to have a working range that depends on particle size and concentration. In binary mixtures and polydisperse solutions, DLS was able to resolve two species in a manner dependent on both concentration and particle size. RMM was able to resolve particles above 200 nm (150 nm for protein) at concentrations below 10(9) particles/mL. In addition, dilution was evaluated as a technique to confirm and quantify the number of particles in solution.