  Roy's equations are used to check if the scalar-isoscalar pion-pion
scattering amplitudes fitted to experimental data fulfill crossing symmetry
conditions. It is shown that the amplitudes describing the ``down-flat'' phase
shift solution satisfy crossing symmetry below 1 GeV while the amplitudes
fitted to the "up-flat'' data do not. In this way the long standing "up-down"
ambiguity in the phenomenological determination of the scalar-isoscalar
pion-pion amplitudes has been resolved confirming the independent result of the
recent joint analysis of the pi+pi- and pi0pi0 data.
