  We consider the dynamics of fermions with a spatially varying mass which
couple to bosons through a Yukawa interaction term and perform a consistent
weak coupling truncation of the relevant kinetic equations. We then use a
gradient expansion and derive the CP-violating source in the collision term for
fermions which appears at first order in gradients. The collisional sources
together with the semiclassical force constitute the CP-violating sources
relevant for baryogenesis at the electroweak scale. We discuss also the absence
of sources at first order in gradients in the scalar equation, and the
limitations of the relaxation time approximation.
