Rheology is demonstrated to be a sensitive and quantitative probe of weak attractive forces acting in concentrated stable colloidal dispersions through comparison of theology and small-angle neutron scattering measurements on a model dispersion with added polyampholyte. Polyampholyte-stabilized dispersions are found to exhibit weak attractions in the form of depletion forces arising from free polyampholyte in the suspending medium. The depletion potential is modeled with the Asakura-Oosawa potential and mapped onto the sticky hard sphere potential to facilitate modeling. Independent validation of the interparticle potential is provided by quantitative prediction of the measured small-angle neutron scattering spectra. A new semiempirical predictive model for the low shear viscosity of stable dispersions is proposed and validated against measurements on model dispersions over a range of compositions. This rheological constitutive relation provides an improved prediction of the low shear viscosity of stable mixtures of adsorbing polyampholyte and colloidal particles, and is anticipated to have broad applicability in modeling and predicting colloidal suspension viscosity. (c) 2005 The Society of Rheology.