This paper deals with the influence on the stress-strain curve of a prestressing steel of such mechanical variables as the gage length, the sample length, and the gripping conditions. An original experimental technique was developed, both to increase the gage length of the standard extensometer and to test large size wires, using different grips. The results show that, at least in a first approach, the stress-strain curve of a prestressing steel has an intrinsic character. Nevertheless, small variations in the yield strength and the flow step were noticed when the sample length changed. These differences may be attributed to the initial curvature of the wire supplied in rolls.