The driving force for the introduction of dislocations at the interface between a strained buffer layer and its substrate is considered. An exact assessment is made of the difference in energy between a system in which prestrain is partially relaxed by the presence of a periodic array of dislocations between layer and substrate and one containing a periodic array which is complete except for one vacant site. The difference provides an expression for the driving force, for introducing the ‘‘last’’ dislocation to complete the periodic array, by the motion of a threading dislocation. All of the dislocation stress fields are accounted for exactly. It emerges that, contrary to intuition, the fluctuating part of the dislocation stress field contributes a term which is attractive rather than repulsive, which is the opposite effect to that found in ‘‘Taylor hardening.’’