  We present a unified string supergravity model based on a string-derived
$SU(5)\times U(1)$ model and a string-inspired supersymmetry breaking scenario
triggered by the $F$-term of the universally present dilaton field. This model
can be described by three parameters: $m_t$, $\tan\beta$, and $m_{\tilde g}$.
We work out the predictions for all sparticle and Higgs masses and discuss the
prospects for their detection at the Tevatron, LEPI,II, and HERA. We find that
the cosmological neutralino relic density is always within current expectations
(\ie, $\Omega_\chi h^2_0\lsim0.9$). We also consider a more constrained version
of this supersymmetry breaking scenario where the $B$-term is specified. In
this case $\tan\beta$ can be determined ($\tan\beta\approx1.4-1.6$) and implies
$m_t\lsim155\GeV$ and $m_h\lsim91\GeV$. Thus, continuing Tevatron top-quark
searches and LEPI,II Higgs searches could probe this restricted scenario
completely.
