  We provide an action for self-dual Yang-Mills theory which is a simple
truncation of the usual Yang-Mills action. Only vertices that violate helicity
conservation maximally are included. One-loop amplitudes in the self-dual
theory then follow as a subset of the Yang-Mills ones. In light-cone gauges
this action is almost identical to previously proposed actions, but in this
formulation the vanishing of all higher-loop amplitudes is obvious; the
explicit perturbative S-matrix is known. Similar results apply to gravity.
