AbstractAccounting for protein flexibility in protein–protein docking algorithms is challenging, and most algorithms therefore treat proteins as rigid bodies or permit side‐chain motion only. While the consequences are obvious when there are large conformational changes upon binding, the situation is less clear for the modest conformational changes that occur upon formation of most protein–protein complexes. We have therefore studied the impact of local protein flexibility on protein–protein association by means of rigid body and torsion angle dynamics simulation. The binding of barnase and barstar was chosen as a model system for this study, because the complexation of these 2 proteins is well‐characterized experimentally, and the conformational changes accompanying binding are modest. On the side‐chain level, we show that the orientation of particular residues at the interface (so‐called hotspot residues) have a crucial influence on the way contacts are established during docking from short protein separations of approximately 5 Å. However, side‐chain torsion angle dynamics simulations did not result in satisfactory docking of the proteins when using the unbound protein structures. This can be explained by our observations that, on the backbone level, even small (2 Å) local loop deformations affect the dynamics of contact formation upon docking. Complementary shape‐based docking calculations confirm this result, which indicates that both side‐chain and backbone levels of flexibility influence short‐range protein–protein association and should be treated simultaneously for atomic–detail computational docking of proteins. Proteins 2005. © 2004 Wiley‐Liss, Inc.