By introducing the concept of a nonhardening region in the plastic strain space, a constitutive model is proposed for cyclic plastic loadings between variable, as well as fixed, strain limits. It is assumed that the isotropic hardening of materials does not occur when the plastic strain point moves inside this region after a load reversal. The region expands and translates as cyclic straining proceeds, and when strain limits are fixed, it eventually occupies the cyclic range of plastic strain so as to describe the saturation of cyclic hardening. The phenomena of cyclic relaxation and cyclic creep are also taken into account in the formulation. In the simple case of a linear hardening material, the present theory is verified by comparing predictions with experimental results on type 304 stainless steel under torsional cyclings between variable, as well as fixed, strain limits at room temperature.