  We consider the centrifugal acceleration of charged test particles by
rotating magnetospheres which are widely believed to be responsible for the
relativistic jet phenomenon in active galactic nuclei (AGN). Based on an
analysis of forces the equation for the radial accelerated motion is derived
and an analytical solution presented under the assumption of an idealized
spherical magnetosphere. We show that the rotational energy gain of charged
particles moving outwards along rotating magnetic field lines is limited in
general by (i) inverse-Compton losses in the radiation field of the disk in
which the magnetosphere is anchored and (ii) the breakdown of the
bead-on-the-wire approximation which occurs in the vicinity of the light
cylinder. The corresponding maximum Lorentz factor for electrons is of the
order of a few hundred for the sub-Eddington conditions regarded to be typical
for BL Lacs. In AGN with enhanced accretion rate the acceleration mechanism
seems to be almost inefficient due to increasing inverse-Compton losses.
