The effects of crystallinity, lamellar thickness and stem length on the yield behaviour of a range of bulk crystallized and pressure annealed polyethylenes have been analysed. The materials studied differ greatly with respect to lamellar thickness, stem length and the degree of crystallinity. Measurements of the yield stress were carried out at -60 degrees C and at a strain rate of 3.5 x 10(-3) s(-1), where the materials are known to exhibit elastic-plastic deformation. The results show that, under these conditions, the yield behaviour is a nucleation-controlled process which is accurately modelled in terms of crystal plasticity, with the yield stress being determined by the stem length of the chains in the crystalline lamellae. Detailed measurements of the stem length and crystallinity have shown that, for these materials, there is an interrelationship between the crystallinity and the stem length. This interrelationship, combined with the crystal plasticity model, leads to the prediction that the yield stress increases as a power-law relationship with crystallinity.