The paper presents the findings of experimental studies on elastic-plastic deformation under complex- variable passive loading of commercially pure nickel samples. The analysis of experimental errors is performed and methods to improve test accuracy are proposed. The properties of the field of plastic compliances are investigated under passive loading. For all loading paths the shapes of equal plastic compliance are close to circles. On the basis of the obtained results, the constitutive equations of multisurface plasticity theory with one active surface of plastic compliances are suggested. A method for determining the model constants, characterizing the evolution of the center of the active surface is proposed based on the experiments under nonproportional complex loading, consisting of primary loading, unloading, secondary loading in a different direction and unloading. The verification of the proposed constitutive equations is performed on the basis of the experiments on thin-walled tubular specimens under complex (monotonic and cyclic) loadings, including partial and intermediate unloading.