  With a detail microscopic model for a self-propelled swimmer, we derive the
rheological properties of a dilute suspension of such particles at small Peclet
numbers. It is shown that, in addition to the Einstein's like contribution to
the effective viscosity, that is proportional to the volume fraction of the
swimmers, a contribution due to the activity of self-propelled particles
influences the viscosity. As a result of the activity of swimmers, the
effective viscosity would be a lower (higher) than the viscosity of the
suspending medium when the particles are pushers (pullers). Such activity
dependent contribution, will also results to a non-Newtonian behavior of the
suspension in the form of normal stress differences.
