We demonstrate that the model of Ball et al. [Polymer 22, 1010 (1981)] has the property of forming a model of a necking polymer. Numerical models of necked bodies are generated by incorporating the theory into a finite element scheme. Large multiaxial extensions are applied to polypropylene sheets at a high temperature to produce necked specimens. Values of the model parameters are chosen such that the observed necks-in uniaxial, planar, and equibiaxial extension-are predicted realistically. The drawing forces observed are also shown to be consistent with the predictions.