The Nyquist or fluctuation-dissipation theorem applies strictly only to linear systems in thermal equilibrium. However, it can also be used successfully to calculate fluctuating quantities in systems in quasi-equilibrium, a familiar example being a circuit with elements at different temperatures. An analogous calculation can be carried out for a plasma, considering each oarticle species as a separate element of the system. One calculates the spontaneous force generated by each species (with its own temperture and mean drift velocity) and also a set of transfer functions which relate each of these forces to the electron perturbations which are responsible for the scattering. The important point to realize in the plasma case is that, with this approach, the electric field perturbations, which couple the species together, enter the calculation only through the transfer function and are not involved in the statistics of the 'generalized forces.' The results obtained apply for multiple ion species with unequal temperatures, unequal mean drift velocities, unequal charges (including negative), etc., and agree with similar calculations based on the dressed particle approach.