The kinetic stability of equilibria derived from distribution functions of the form F+or-(H,Ptheta ), is analysed. The considered class of perturbations are constant along the theta -direction. A variational principle for internal modes is derived. In particular, the orbit integrals are eliminated in this formalism. Under weak assumptions, it is possible to show that the kinetic stability criterion is fulfilled for MHD stable configurations. The study is relevant to 'low- beta ' systems like Tokamaks and theta -pinches. However, the results have most interest for devices with magnetic nulls, such as Extrap and Multipoles. The results can also be applied to purely radial displacements in RFPs.