1. With fixed constants in the Manson-Coffin equation, the greatest endurance is obtained in complex stress-strain for arbitrary — including nonproportional — cyclic loading paths when the equivalent plastic-strain range is determined from an analog of the Tresca-Saint-Venant criterion. Increasingly lower values of endurance are obtained when use is made of the theory of maximum strains, analogs of the von Mises criterion, and kinetic damage criteria, respectively. 2. The better-known empirical data on low-cycle fatigue under nonproportional loading is described best when the equivalent plastic-strain range is determined from a kinetic criterion. 3. Within the framework of the kinetic criterion, endurance is always lower under nonproportional cyclic deformation than in the case of proportional deformation having the same diameter of strain path per cycle.