  Relativistic arbitrary spin Hamiltonians are shown to obey the algebraic
structure of supersymmetric quantum system if their odd and even parts commute.
This condition is identical to that required for the exactness of the
Foldy-Wouthuysen transformation. Applied to a massive charged spin-$1$ particle
in a constant magnetic field, supersymmetric quantum mechanics necessarily
requires a gyromagnetic factor $g=2$.
