The analysis of strain-hardening materials subjected to multiaxial states of stress requires more detailed experimental information about the effects of previous plastic deformation on the yield surfaces of real materials than is presently available. To provide insight into some of these effects thin walled tubular specimens of 1100-0 aluminum and annealed OFHC copper were subjected to biaxial stresses through the application of simultaneous axial tension and internal pressure, and the effects of the magnitude, direction, and sequence of prestraining operations on subsequent yield surfaces were determined. It was found that the yield surface behavior depends greatly upon the definition of yielding employed. Use of small proof strain definitions resulted in very anisotropic yield surface characteristics which reflected the effect of previous deformation. On the other hand, use of large proof strains resulted in isotropic yield surface characteristics which were devoid of previous deformation influence. The small proof strain yield curves were found, in general, to expand and translate in the direction of prestrain and, for biaxial prestrains, to be distorted in the vicinity of the loading point. Multiple prestrain sequences in normal directions induce a large negative cross effect similar to Bauschinger effect observed under reversed loading. Such anisotropic behavior was found to contradict the two most commonly used continuum mechanics predictions, the isotropic and kinematic hardening rules.