Enhanced production of strange quarks is considered one of the signatures of a transient quark-gluon-plasma stage in ultrarelativistic nucleus-nucleus collisions. From the measurement of strange and non-strange particle production rates one can deduce the content of strange (s+sbar) and non-strange (u+ubar,d+dbar) quarks in the hadronic final state of the reaction. Expressed in terms of the ratio lambda_s= 2./(+) the experimental results from central Pb and S beam interactions with nuclei at the SPS (red dots) show that strangeness production is indeed enhanced by a factor of two with respect to the elementary e+e-, pp, ppbar, and pA collisions (blue triangles,squares). Strangeness enhancement is seen to the same degree in Pb+Pb as in S+nucleus reactions.