In this paper a family of dilatant plasticity theories is introduced by considering yield surfaces which change according to a combination of isotropic expansion and kinematic translation. One limiting member of the family is Gurson's (1977) isotropic hardening model, and the other limiting member is a pure kinematic hardening version. The family of constitutive laws is constructed such that all versions coincide for proportional stressing histories. The differences between any two versions show up only under nonproportional stressing histories, such as those encountered in many plastic instability phenomena. Under nonproportional stressing, the kinematic version is significantly "less stiff" than Gurson's isotropic hardening model due to the relatively higher curvature of the kinematic yield surface. This effect is explored in some basic shear localization calculations and is found to have substantial influence on the localization predictions.