  Centrifugal acceleration of charged test particles at the base of a rotating
jet magnetosphere is considered. Based on an analysis of forces we derive the
equation for the radial accelerated motion and present an analytical solution.
It is shown that for particles moving outwards along rotating magnetic field
lines, the energy gain is in particular limited by the breakdown of the
bead-on-the-wire approximation which occurs in the vicinity of the light
cylinder $r_{L}$. The corresponding upper limit for the maximum Lorentz factor
$\gamma_{max}$ for electrons scales $\propto B^{2/3} r_{L}^{2/3}$, with $B$ the
magnetic field strength at $r_{L}$, and is at most of the order of a
$10^2-10^3$ for the conditions regarded to be typical for BL Lac objects. Such
values suggest that this mechanism may provide pre-accelerated seed particles
which are required for efficient Fermi-type particle acceleration at larger
scales in radio jets.
