  We consider massless higher spin gauge theories with both electric and
magnetic sources, with a special emphasis on the spin two case. We write the
equations of motion at the linear level (with conserved external sources) and
introduce Dirac strings so as to derive the equations from a variational
principle. We then derive a quantization condition that generalizes the
familiar Dirac quantization condition, and which involves the conserved charges
associated with the asymptotic symmetries for higher spins. Next we discuss
briefly how the result extends to the non linear theory. This is done in the
context of gravitation, where the Taub-NUT solution provides the exact solution
of the field equations with both types of sources. We rederive, in analogy with
electromagnetism, the quantization condition from the quantization of the
angular momentum. We also observe that the Taub-NUT metric is asymptotically
flat at spatial infinity in the sense of Regge and Teitelboim (including their
parity conditions). It follows, in particular, that one can consistently
consider in the variational principle configurations with different electric
and magnetic masses.
