Extensive experimental data indicates that the particle property dependence of the just-suspended agitation speed of pitched-blade turbines and high-efficiency impellers can be described with reasonable accuracy using a Zwietering power-law correlation. However, there are indications that the effect of particle size is divided into two regimes, one for particle sizes less than 1000 mu m and one for larger particles. Comparison of the two impeller types indicates that the pitched-blade turbine requires somewhat higher power inputs and substantially higher torque levels to produce the same level of agitation as the high-efficiency impeller.