Applied error fields raise the H-mode power threshold, but do not significantly affect energy confinement in the JFT-2M tokamak. Magnetic perturbations of magnitude n.m/BT ≈ (2-5) × 10−4 resonant near the plasma edge, raise the H-mode power threshold from 350 kW to 550 kW of neutral beam injection power. The global energy confinement is not affected once the H-mode is achieved. These measurements give an indication of the accuracy to which tokamak coils should be designed and maintained in order to ensure optimal access to the H-mode.