  Stationary axisymmetric magnetic fields are expelled from outer horizons of
black holes as they become extremal. Extreme black holes exhibit Meissner
effect also within exact Einstein--Maxwell theory and in string theories in
higher dimensions. Since maximally rotating black holes are expected to be
astrophysically most important, the expulsion of the magnetic flux from their
horizons represents a potential threat to an electromagnetic mechanism
launching the jets at the account of black-hole rotation.
