The behavior of suspensions of rigid particles in a non-Newtonian fluid is studied in the framework of a nonlinear homogenization method. Estimates for the overall properties of the composite material are obtained. In the case of a Herschel-Bulkley suspending fluid, it is shown that the properties of a suspension with overall isotropy can be satisfactorily modeled as that of a Herschel-Bulkley fluid with an exponent equal to that of the suspending fluid. Estimates for the yield stress and the consistency at large strain rate levels are proposed. These estimates compare well to both experimental data obtained by Mahaut [J. Rheol. 52(1), 287-313 (2008)] and to experimental data found in the literature. (C) 2008 The Society of Rheology.