  We discuss the predictions of S-duality for the monopole spectrum of
four-dimensional heterotic string theory resulting from toroidal
compactification. We discuss in detail the spectrum of "H-monopoles", states
that are magnetically charged with respect to the U(1) groups arising from the
dimensional reduction of the ten dimensional antisymmetric tensor field. Using
an assumption concerning the correct treatment of collective coordinates in
string theory we find results which are consistent with S-duality.
