Biaxial test methods have been used to determine, not only yield behaviour under biaxial conditions, but also the strain response. This paper examines the influence of uniaxial prestrain upon the biaxial r‐value by extending the disc compression test procedure proposed by Barlat et al. . The extension involved the use of digital image measurements of in‐plane strains. The material examined was a 7075‐O condition aluminium alloy. The results of the experimental programme indicated that the biaxial r‐value is unaffected by uniaxial prestrain. When using the disc compression test, the mode of deformation and therefore the biaxial r‐value were found to be very sensitive to the prevailing friction conditions.