  We investigate the production of circularly polarised X and gamma-ray signals
in cosmic accelerators such as supernova remnants and AGN jets. Proton-proton
and proton-photon collisions within these sites produce a charge asymmetry in
the distribution of mesons and muons that eventually leads to a net circular
polarisation signal as these particles decay radiatively. We find that the
fraction of circular polarisation thus produced is at the level of $ 5 \times
10^{-4}$, regardless of the exact beam spectrum, as long as it is made
predominantly of protons. While this fraction is very small, the detection of
circular polarisation signals in conjunction with high-energy neutrinos would
provide an unambiguous signature of the presence of high-energy protons in
cosmic accelerators. In Supernovae shocks in particular, this would indicate
the presence of relativistic protons hitting stationary protons and/or
low-energy photons in the intergalactic or interstellar medium.
