  Based on tree-level open-string scattering amplitudes in the low string-scale
scenario, we derive the massless fermion scattering amplitudes. The amplitudes
are required to reproduce those of the Standard Model at tree level in the low
energy limit. We then obtain four-fermion contact interactions by expanding in
inverse powers of the string scale and explore the constraints on the string
scale from low energy data. The Chan-Paton factors and the string scale are
treated as free parameters. We find that data from the neutral and charged
current processes at HERA, Drell-Yan process at the Tevatron, and from LEP-II
put lower bounds on the string scale M_S >= 0.9-1.3 TeV for typical values of
the Chan-Paton factors, which are comparable to Tevatron bounds on Z' and W'
masses.
