  We study suspensions of deformable (viscoelastic) spheres in a Newtonian
solvent in plane Couette geometry, by means of direct numerical simulations. We
find that in the limit of vanishing inertia the effective viscosity $\mu$ of
the suspension increases as the volume-fraction occupied by the spheres $\Phi$
increases and decreases as the elastic modulus of the spheres $G$ decreases;
the function $\mu(\Phi,G)$ collapses to an universal function, $\mu(\Phi_e)$,
with a reduced effective volume fraction $\Phi_e(\Phi,G)$. Remarkably, the
function $\mu(\Phi_e)$ is the well-known Eilers fit that describes the rheology
of suspension of rigid spheres at all $\Phi$. Our results suggest new ways to
interpret macro-rheology of blood.
