A theoretical treatment of particle‐particle interaction is described from which the viscosity‐concentration behavior of multimodal suspensions of rigid particles can be related to the viscosity‐concentration behavior of the unimodal components. From this theory, the viscosity of multimodal suspensions can be calculated and shows excellent agreement with existing experimental data. Blend ratios that will produce minimum viscosities are simply derived from the theory and agree well with experimental results. Another important feature of this theory is that it predicts and defines a lower limit for the viscosity at any concentration and indicates that this lowest viscosity can be obtained with a variety of solids combinations.