An original technique for the determination of the fraction of unsuspended solid particles at agitation speeds smaller than N-js, is proposed. The technique is based on the "twin-systems" concept and is completed by a suitable tracing and detection technique for the solid phase. Results obtained with a fully baffled tank stirred by a Rushton turbine impeller show that the fraction of solids never leaving the tank bottom can be simply correlated to the ratio between agitation speed and Zwietering＇s complete suspension speed. It is shown that at agitator speeds of about 80% of Zwietering＇s complete suspension speed, practically all particles but a few, are already suspended. This finding implies that large energy and installation cost savings can be achieved, with respect to current design practices, without significantly affecting the solid-liquid mixing performance.