Mechanical and structural analyses have been carried out on a range of bulk-cast polyethylenes, which differ widely with respect to their stem lengths (the length traversed by the polymer chains within the crystalline lamellae). The effects of temperature and stem length on the yield stress were evaluated at a strain-rate of 10(-2) s(-1), over a temperature range from -60 to 60 degrees C; a temperature range which includes the beta-relaxation temperature for these materials. The results have shown that using a modified crystal plasticity approach, which takes into account the dislocation core energy, the temperature and stem length dependence of the yield stress can be modelled concurrently. This has not previously been attempted. The modified crystal plasticity approach is found to be valid both below the beta-relaxation temperature, under conditions of elastic-plastic deformation; and above the beta-relaxation temperature under conditions of viscoelastic deformation.