Transverse mass spectra of pions, kaons, and protons from the symmetri
c heavy-ion collisions 200A GeV S + S and 158A GeV Pb + Pb, measured i
n the NA44 focusing spectrometer at CERN, are presented. The mass depe
ndence of the slope parameters provides evidence of collective transve
rse flow from expansion of the system in heavy-ion induced central col
lisions.