It has been established by computation, and confirmed by analysis, for an infinite slab of strain-gradient sensitive material subjected to plane-strain tensile loading, that passivation of the lateral boundaries at some stage of loading inhibits plastic deformation upon further loading. This result is not surprising in itself except that, remarkably, if the gradient terms contribute to the dissipation, the plastic deformation is switched off completely, and only resumes at a clearly-defined higher load, corresponding to a total strain ε_c say. The analysis presented in this paper confirms the delay of plastic deformation following passivation and determines the exact manner in which the plastic flow resumes. The plastic strain-rate is continuous at the exact point ε_c of resumption of plastic flow and, for the first small increment Δε = ε − ε_c in the imposed total strain, the corresponding increment in plastic strain, Δε^p, is proportional to (Δε)^2. The constant A in the relation Δε^p(0) = A(Δε)^2, where Δε^p(0) denotes the plastic strain increment at the centre of the slab, has been determined explicitly; it depends on the hardening modulus of the material. The presence of energetic gradient terms has no effect on the value of ε_c unless the dissipative terms are absent, in which case passivation reduces the rate of plastic deformation but introduces no delay. This qualitative effect of dissipative gradient terms opens the possibility of experimental discrimination of their presence or absence. The analysis employs an incremental variational formulation that is likely to find use in other problems.