  We consider the structure factor of a system of colloidal particles immersed
in a host liquid. Each particle is assumed to be affected by forces due to
other particles, a drag force proportional to the velocity of the particle
relative to the local velocity of the fluid and a fluctuating random noise. The
effect of the particles on the velocity field of the host liquid is neglected.
The problem is treated within the RPA approximation which is a widely used tool
in many body theory. The validity of the RPA result is discussed.
