The interest in the production of fibres that mimic the behaviour of natural silks has been boosted by the first successful attempts of spinning fibres based on spider drag line silk proteins. However, both the processing of biomimetic silk fibres and the basic studies on silk are hampered by the large variability of the fibre properties. Here we show that the tensile behaviour of spider silk can be predictably and reproducibly tailored by controlling the supercontraction effect, a large shrinkage of the longitudinal dimension of the fibre if unrestrained by its ends and immersed in water. This procedure allows to reproduce the tensile behaviour of natural drag line fibres and offers the possibility of obtaining silk fibres with predictable tailored properties in large quantities for experimental use. These results can be interpreted in the frame of the molecular model of drag line silk, as the result of re-orientation of the protein chains, leading to an explanation for the observed variability of natural drag line fibres. (C) 2003 Elsevier Science Ltd. All rights reserved.