A recent extension of the theory of ferromagnetic hysteresis to include the effects of a constant applied stress uses an effective field H/sub sigma / that depends on the stress and the rate of change of magnetostriction with magnetization. The extended theory is used here to derive a particularly simple expression for the variation with stress of the anhysteretic differential susceptibility at M=O, H=O. The theoretical prediction is compared with experimental results and shows excellent agreement. This expression forms the basis for a novel technique of evaluating stress in ferromagnetic steels. >