The interaction of microwaves with a magnetoactive plasma is investigated with a plasma loaded resonant cavity. The cylindrical cavity resonant in the TE[111] mode has a coaxial plasma rod which is longitudinally magnetised. When empty the cavity supports two circularly polarised modes with the same resonant frequency and the introduction of the magnetoactive plasma removes the degeneracy. The system is analysed firstly by the perturbation theory which has a number of limitations. Some of these are removed by a second method of analysis in which the propagation characteristics of a waveguide with three coaxial media are determined. This analysis is restricted to a lossless plasma but allows the perturbation theory to be modified and thus applied to the lossy system. The experimental observations on the changes in resonant frequency and Q-value of the cavity caused by the plasma are interpreted with the theory to give values of the plasma characteristics, electron density and collision frequency.