  It is shown that data on pion and strangeness production in central
nucleus-nucleus collisions are consistent with the hypothesis of a Quark Gluon
Plasma formation between 15 A GeV/c (BNL AGS) and 160 A GeV/c (CERN SPS)
collision energies. The experimental results interpreted in the framework of a
statistical approach indicate that the effective number of degrees of freedom
increases by a factor of about 3 in the course of the phase transition and that
the plasma created at CERN SPS energy may have a temperature of about 280 MeV
(energy density $\approx$ 10 GeV/fm^3). Experimental studies of central Pb+Pb
collisions in the energy range 20-160 A GeV/c are urgently needed in order to
localize the threshold energy, and study the properties of the QCD phase
transition.
