  We compare the host galaxies properties of BL Lac objects with those of radio
loud quasars (RLQs) imaged by the WFPC2 on board of HST. The considered objects
(z<0.5) are always well resolved and their host galaxies satisfactorily
modelled by ellipticals. After homogeneous treatment of the data we find RLQs
hosts are systematically more luminous (by ~0.7 mag) with respect to the hosts
of BL Lacs. Using the M_BH - L_bulge relation, derived for nearby elliptical
galaxies, we have evaluated the central black hole masses of our sample of
active galaxies. These data are discussed in conjunction with the nuclear
luminosity and the Eddington ratio.
