We discuss the properties of the sources in the CLASS blazar survey, which aims at the selection of low radio power (P_5GHz_<10^25^W/Hz) blazars. We use Very Large Array (VLA) data from available catalogues and from our own observations to constrain the radio core-dominance of the sample which, together with the flat radio spectral index, is a signature of the blazar activity. X-ray data from the ROSAT All Sky Survey were also collected in order to constrain the radio-to-X-ray luminosity ratio of the sources.