  We propose a model to describe the concentration dependence of the viscosity
of soft particles. We incorporate in a very simple way the softness of the
particles into expressions originally developed for rigid spheres. This is done
by introducing a concentration-dependent critical packing, which is the packing
at which the suspension looses fluidity. The resultant expression reproduces
with high accuracy the experimental results for suspensions of star polymers in
good solvents. The model allows to explain a weak increase of the viscosity
observed in the case of diblock copolymer stars suggesting that the reason for
this peculiar behavior is mainly a consequence of the softness of the
particles. In the semi-dilute regime, suspensions of star polymers are modeled
using the Daoud-Cotton picture to complete the description in the whole
concentration regime.

Soft Condensed Matter