For fluctuations in thermal equilibrium we have the fluctuation-dissipation theorem as a fundamental and general result valid not only for plasma media. The analysis based on the dressed particle model should reproduce the results of this theorem when it is applied to plasmas with Maxwellian velocity distributions. With this test fulfilled, it is then justifiable to use the dressed particle model also for plasmas out of thermal equilibrium, although the plasma is required to be stable with respect to perturbations.