  The helicity of a free massless relativistic particle, $\chi^o={\bf s}.{\bf
p}/|{\bf p}|$, is generalized, for a particle in an electromagnetic field, to
$\chi={\bf s}.{\bf p}/{\cal E}$, where ${\cal E}$ is the modified kinetic
energy. Both $\chi^o$ and $\chi$ coincide and are conserved for minimal
coupling (gyromagnetic ratio $g=0$) but are different and neither of them is
conserved when the coupling is non-minimal, $g\neq0$, generating non-zero
effective mass. For a chiral particle with $g=2$ in a constant electric field
both helicities converge asymptotically to the same value. Helicity is also
conserved for minimal gravitational coupling.
