  We present new optical circular polarization measurements with typical
uncertainties < 0.1% for a sample of 21 quasars. All but two objects have null
circular polarization. We use this result to constrain the polarization due to
photon-pseudoscalar mixing along the line of sight. We detect significant (> 3
sigma) circular polarization in two blazars with high linear polarization and
discuss the implications of this result for quasar physics. In particular, the
recorded polarization degrees may be indicative of magnetic fields as strong as
1 kG or a significant contribution of inverse Compton scattering to the optical
continuum.
