The effect of particles on the development of instability patterns during the capillary thinning process of a poly(ethylene oxide) solution has been experimentally studied in an effort to clarify the filament breakup behavior of particulate systems. We focus on breathing and formation of the first bead in particular, which as this bead initiate blistering. Changes in instability patterns due to an addition of PMMA particles are investigated by observing the evolution of a filament undergoing capillary thinning. The addition of particles accelerates breathing and the formation of the first bead. By comparing the extensional properties of a PMMA/PEO suspension to those of glycerin/PEO solutions which has the same shear viscosity, surface tension and polymer concentration, it was found that the addition of particles has little effect on the behavior of polymer chains under an extensional flow. Meanwhile, the flow becomes heterogeneous with the fluctuation of the local particle concentration, inducing rapid thinning at the neck region, which propagates throughout the filament as capillary thinning proceeds. The addition of particles also influences the position of the first bead. For a pure polymer solution and a polymer solution with a small amount of particles (similar to 2 wt.%), the first bead mostly appears near the upper neck. However, as the particle concentration increases to 10 wt.%, the first bead appears at random locations along the filament, with similar probabilities. The randomization of the position of the first bead is attributed to the non-uniform distribution of particles inside a thinning filament due to the fluctuation of the local particle concentration.