  We report the discovery of circularly polarised radio emission from the
radio-jet X-ray binary SS 433 with the Australia Telescope Compact Array. The
flux density spectrum of the circular polarization, clearly detected at four
frequencies between 1 - 9 GHz, has a spectral index of (-0.9 +/- 0.1). Multiple
components in the source and a lack of very high spatial resolution do not
allow a unique determination of the origin of the circular polarization, nor of
the spectrum of fractional polarization. However, we argue that the emission is
likely to arise in the inner regions of the binary, possibly via
propagation-induced conversion of linear to circular polarization, and the
fractional circular polarization of these regions may be as high as 10%.
Observations such as these have the potential to investigate the composition,
whether pairs or baryonic, of the ejecta from X-ray binaries.
