  Scalar cosmological perturbations of a weakly self-interacting plasma mixed
with a perfect radiation fluid are investigated. Effects of this plasma are
considered through order $\lambda^{3/2}$ of perturbative thermal-field-theory
in the radiation dominated universe. The breakdown of thermal perturbation
theory at vastly subhorizon scales is circumvented by a Pad\'e approximant
solution. Compared to collisionless plasmas the phase speed and subhorizon
damping of the plasma density perturbations are changed. An example for a
self-interacting thermal field is provided by the neutrinos with effective
4-fermion interactions.
