At Princeton McKellar began his work on weak interactions, calculating the parity violation in expected in nuclei. He also started his work on three and many body nuclear forces. It was characteristic of this work that he was applying the current algebra techniques of particle physics to obtain results about nuclei. During his time at the University of Melbourne, McKellar and his collaborators published his definitive study on three nucleon forces, famously known as the "Tucson-Melbourne" force. His work on weak interactions led to calculation of the electric dipole moments expected for the nucleon and atoms in various models of these interactions. This work then evolved into studies of related effects in the B meson system.