  The Barut--Zanghi (BZ) theory can be regarded as the most satisfactory
picture of a classical spinning electron and constitutes a natural "classical
limit" of the Dirac equation. The BZ model has been analytically studied in
some previous papers of ours in the case of free particles. By contrast, in
this letter we consider the case of external fields, and a previously found
equation of the motion is generalized for a non-free spin-1/2 particle. In the
important case of a spinning charge in a uniform magnetic field, we find that
its angular velocity (along its circular orbit around the magnetic field
direction) is slightly different from the classical "cyclotron frequency" eH/m
which is expected to hold for spinless charges. As a matter of fact, the
angular velocity results to depend on the spin orientation. As a consequence,
the electrons with magnetic moment mu parallel to the magnetic field do rotate
with a frequency greater than that of electrons endowed with mu antiparallel to
H.
