A large set of high-precision, full-Stokes spectro-polarimetric observations of prominences in He I D3, Hα, and Hβ lines has been recorded with the ZIMPOL polarimeter at the Gregory-Coudé Telescope in Locarno. The observational technique allows us to obtain measurements free from seeing-induced spurious effects. The instrumental polarization is well under control and taken into account in the data analysis. We present our observational results for each of the lines mentioned above. Of particular interest is that most of our Hα measurements show antisymmetric V profiles that are a characteristic signature of the Zeeman effect in the prominence plasma. A Stokes inversion technique based on the quantum theory of the Hanle and Zeeman effects is being applied on observed Stokes profiles in the He I D3 line in order to obtain information on the magnetic field vector that confines the prominence plasma.