  We describe some ambiguities which take place when on calculates the
cross-sections in parton models at high energies and the connected limitations
on the asymptotic of high energy amplitudes that follows from the conditions of
boost-invariance of cross-sections.
  It turns out that the resulting constraints are of the same type as the
following from the t-channel unitarity conditions.~ So that on can suppose that
this similarity, by their nature, has much more general grounds.
