  We study how quasiparticle interactions affect their localization properties
in dirty superconductors with broken time reversal symmetry -- for example in a
magnetic field. For SU(2) spin-rotation invariant (class C) systems, the only
important coupling is the spin-spin triplet interaction, which we study within
a renormalization group approach. Either an additional Zeeman coupling or a
complete breaking of spin rotation symmetry renders all interactions
irrelevant. These two situations realize, respectively, the non-interacting
unitary Anderson and the ``thermal'' (class D) universality class. Our results
imply a stable metallic phase in 2D for class D. Experimental implications are
discussed.
