The constitutive response of a Sigma9(221) symmetric tilt grain boundary in copper is investigated with an atomistic quasicontinuum method under tension and shear. The maximum strength is found to change with the deformation mode triggered at the interface. Atom shuffling processes cause a major decrease in the intrinsic boundary strength under shear. (C) 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.