The response of a deformable solid, in which dislocations are assumed to be highly mobile, to the presence of a loaded crack has been examined, and two cases have been considered. In the first case, that of a crack which pre-exists at zero load, it was confirmed that dislocation emission from the crack tip always precludes brittle crack propagation. In the second case, a microcrack is injected into the loaded deformable solid, for example, due to the cracking of a brittle inclusion. In this case simultaneous dislocation emission and brittle crack propagation can occur, depending on the cohesive energy and the dislocation mobility. Both cases have been studied dynamically, assuming fast moving dislocations, and the effect of the presence of dislocations upon the crack-tip field was fully taken into account. The implications of these results for understanding the brittle-ductile transition and impurity-induced intergranular embrittlement are discussed.