  We investigate the optical spectral properties of the blazar PKS 1222+216
during a period of 3 years. While the continuum is highly variable the broad
line emission is practically constant. This supports a scenario in which the
broad line region is not affected by jet continuum variations. We thus infer
the thermal component of the continuum from the line luminosity and we show
that it is comparable with the continuum level observed during the phases of
minimum optical activity. The mass of the black hole is estimated through the
virial method from the FWHM of MgII, Hbeta, and Halpha broad lines and from the
thermal continuum luminosity. This yields a consistent black hole mass value of
6x10^8 solar masses.
