The geometrical structure and the quantum properties of the recently proposed harmonic-space action describing self-dual Yang-Mills (SDYM) theory are analyzed. The geometrical structure that is revealed is closely related to the twistor construction of instanton solutions. The theory gets no quantum corrections and, despite having SDYM as its classical equation of motion, its S-matrix is trivial. It is therefore not the theory of the N = 2 string. We also discuss the five-dimensional actions that have been proposed for SDYM.