  The magnetic field in an accretion disk is estimated assuming that all of the
angular momentum within prescribed accretion disk radii is removed by a jet.
The magnetic field estimated at the base of the jet is extrapolated to the
blazar emission region using a model for a relativistic axisymmetric jet
combined with some simplifying assumptions based on the relativistic nature of
the flow. The extrapolated magnetic field is compared with estimates based upon
the synchrotron and inverse Compton emission from three blazars, MKN 501, MKN
421 and PKS 2155-304. The magnetic fields evaluated from pure synchrotron self-
Compton models are inconsistent with the magnetic fields extrapolated in this
way. However, in two cases inverse Compton models in which a substantial part
of the soft photon field is generated locally agree well, mainly because these
models imply magnetic field strengths which are closer to being consistent with
Poynting flux dominated jets. This comparison is based on estimating the mass
accretion rate from the jet energy flux. Further comparisons along these lines
will be facilitated by independent estimates of the mass accretion rate in
blazars and by more detailed models for jet propagation near the black hole.
