of about 4 GeV. Any local fluctuation more massive than this would grow rapidly, converting the surrounding superheated hadronic matter to quark-gluon plasma. A similar estimate, based on the MIT bag model, a simpler hadronic equation of state (free pion gas), and with zero baryon density, was obtained much earlier [@old]. Another estimate is obtained by the argument that at these relatively modest beam energies most meson production occurs through the formation and decay of baryon resonances: $\Delta$, $N^*$, etc. The most massive observed resonances are in the range of 2 to 2.5 GeV. Putting two of these in close physical proximity leads to a mass of 4 to 5 GeV. We now need an estimate of the number of pions this critical mass corresponds to. Let us assume that each particle, nucleon and meson, carries away a kinetic energy equal to one half its rest mass. If a particle would have too great a kinetic energy then it might escape from the nucleon-nucleon collision volume long before its neighbors and so would not be counted in the rest mass of the local fluctuation. Taking 4 GeV, dividing by 1.5, and subtracting twice the nucleon mass leaves about 6 pion 