A quantitative description of plastic deformation in crystalline solids requires a knowledge of how an assembly of dislocations-the defects responsible for crystal plasticity-evolves under stress(1). In this context, molecular-dynamics simulations have been used to elucidate interatomic processes on microscopic (similar to 10(-10) m) scales(2), whereas ＇dislocation-dynamics＇ simulations have explored the long-range elastic interactions between dislocations on mesoscopic (similar to 10(-6) m) scales(3). But a quantitative connection between interatomic processes and behaviour on mesoscopic scales has hitherto been lacking. Here we show how such a connection can be made using large-scale (100 million atoms) molecular-dynamics simulations to establish the local rules for mesoscopic simulations of interacting dislocations. In our molecular-dynamics simulations,we observe directly the formation and subsequent destruction of a junction (a Lomer-Cottrell lock) between two dislocations in the plastic zone near a crack tip : the formation of such junctions is an essential process in plastic deformation, as they act as an obstacle to dislocation motion. The force required to destroy this junction is then used to formulate the critical condition for junction destruction in a dislocation-dynamics simulation, the results of which compare well with previous deformation experiments(4).