  We construct tree-level four-particle open-string amplitudes relevant to
dilepton and diphoton production at hadron colliders. We expand the amplitudes
into string resonance (SR) contributions and compare the total cross-section
through the first SR with the $Z^{\prime}$ search at the Tevatron. We establish
a current lower bound based on the CDF Run I results on the string scale to be
about $1.1-2.1$ TeV, and it can be improved to about $1.5-3$ TeV with 2
fb$^{-1}$. At the LHC, we investigate the properties of signals induced by
string resonances in dilepton and diphoton processes. We demonstrate the unique
aspects of SR-induced signals distinguishable from other new physics, such as
the angular distributions and forward-backward asymmetry. A 95% C.L. lower
bound can be reached at the LHC for $M_S>8.2-10$ TeV with an integrated
luminosity of 300 fb$^{-1}$. We emphasize the generic features and profound
implications of the amplitude construction.
