In this paper, we estimate the volume density of dislocation sources formed as a result of contact interaction between dislocations slipping in an octahedral plane and Lomer-Cottrell dislocation locks. Assuming that during the deformation each dislocation source forms a slip trace, we estimate the average distance between slip traces. This distance turns out to be two to three orders of magnitude less than the experimentally observed value. We draw the conclusion that dislocation lock sources may ensure plastic deformation of fcc crystals.