[en] It is shown that a reduction of S-matrix elements based on the use of Yang-Feldmann relations leads to non-uniquely defined results even on mass shell. Using generally accepted techniques quite arbitrary restrictions for the behaviour on mass shell can be derived, which is connected with the properties of the reduced elements under space-time reflection. The standard expressions which fulfil the unitarity condition automatically appear so only as a special choice subjected to the same lack of internal consistency. The reduction formulae considered in momentum space turn out to be ill-defined strictly on mass shell and need a further prescription. However, if one uses for the latter a procedure via off-mass-shell extrapolations conclusive results seem only to be achievable if besides the current operator corresponding to a first reduction free fields are employed. (author)