  In the quantized two-dimensional non-linear supersymmetric $\sigma$-model,
the supercurrent supermultiplet, which contains the energy-momentum tensor, is
transformed by the nonlocal symmetry of the model into the isospin current
supermultiplet. This effect incorporates supersymmetry into the known
infinite-dimensional Yangian deformation symmetry of plain $\sigma$-models,
leads to precisely the same nontrivial extension of the two-dimensional
super-Poincar\'e group as found previously for the Poincar\'e group, and thus
determines the theory's mass spectrum. A generalization to all higher-order
nonlocal charges is conjectured such that their generating function, the
so-called ``master charge'', has a definite Lorentz spin which depends on the
spectral parameter.
