Ultra-sensitive detection by accelerator mass spectrometry of the long-lived plutonium isotopes, 239,240,242,244Pu, has been demonstrated at an accelerating voltage of 300 kV. Transmission efficiencies as high as 15% have been observed for Th3+ ions. Using a gas ionisation detector with an ultra-thin silicon nitride window, an energy resolution sufficient to separate Pu3+ ions from interfering Dy2+ ions with 2/3 of the energy was possible. Sensitivities approaching 106 atoms for the various Pu isotopes were achieved.