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Net proton and negative hadron spectra for central Pb 1 Pb collisions at 158 GeV per nucleon at the CERN Super Proton Synchrotron were measured and compared to spectra from lighter systems. Net baryon distributions were derived from those of net protons. Stopping (rapidity shift with respect to the beam) and mean transverse momentum ͗ p T ͘ of net baryons increase with system size. The rapidity density of negative hadrons scales with the number of participant nucleons for nuclear collisions, whereas their ͗ p T ͘ is independent of system size. The ͗ p T ͘ dependence upon particle mass and system size is consistent with larger transverse flow velocity at midrapidity for Pb 1 Pb compared to S 1 S central collisions. Lattice QCD predicts that strongly interacting matter at an energy density greater than 1 2 GeV͞fm 3 attains a deconfined and approximately chirally restored state known as the quark-gluon plasma (for an overview, see [1]). This state of matter existed in the early Universe, and it may influence the dynamics of rotating neutron stars [2]. The collision of nuclei at ultrarelativistic energies offers the possibility in the laboratory of creating strongly interacting matter at sufficiently high energy density to form a quark-gluon plasma [3]. Hadronic spectra from these reactions reflect the dynamics of the hot and dense zone formed in the collision. The baryon density, established 0031-9007͞99͞82(12)͞2471(5)$15.00