  We determine the entropy per baryon content of the central reaction region in
terms of the charged particle multiplicity. We study the consistency of our
findings with recent data on strange anti-baryon production at 200 GeV A in S
-> A collisions (A=200) assuming formation of a central fireball. Hadron gas
models which do not invoke strong medium modifications of hadron masses do not
provide enough entropy and are inconsistent with the combined experimental
results. In contrast the quark-gluon plasma hypothesis explains them naturally.
