Cell models for the viscosity mu of a suspension of spherical particles in Newtonian fluid of viscosity mu(0) are discussed. Various boundary conditions on the outer boundary of the spherical cell lead to different results: the viscosity predicted for a zero velocity perturbation boundary condition is higher than those predicted with zero tangential stress perturbation. However, in all cases Einstein's prediction mu/mu(0) = 1 + 2.5 phi+(...) holds in the limit of small solid volume fraction phi << 1. (c) 2006 Elsevier Ltd. All rights reserved.