  We describe an algorithm that computes the linear dispersion relation of
waves and instabilities in relativistic plasmas within a Vlasov-Maxwell
description. The method used is fully relativistic and involves explicit
integration of particle orbits along the unperturbed equilibrium trajectories.
We check the algorithm against the dispersion curves for a single component
magnetised plasma and for an unmagnetised plasma with counter-streaming
components in the non-relativistic case. New results on the growth rate of the
Weibel or two-stream instability in a hot unmagnetised pair plasma consisting
of two counter-streaming relativistic Maxwellians are presented. These are
relevant to the physics of the relativistic plasmas found in gamma-ray bursts,
relativistic jets and pulsar winds.
