We explore the one-loop electroweak radiative corrections in the context of the traditional minimal $SU(5)$ and the string-inspired $SU(5)\times U(1)$ supergravity models by calculating explicitly vacuum-polarization and vertex-correction contributions to the $��_1$ and $��_b$ parameters. We also include in this analysis the constraint from $b\rightarrow s��$ whose inclusive branching ratio $B(b\rightarrow s��)$ has been actually measured very recently by CLEO. We find that by combining these three most important indirect experimental signatures and using the most recent experimental values for them, $m_t\gtrsim 170 {\rm GeV}$ is excluded for $��>0$ in both the minimal $SU(5)$ supergravity and the no-scale $SU(5)\times U(1)$ supergravity. We also find that $m_t\gtrsim 175(185) {\rm GeV}$ is excluded for any sign of $��$ in the minimal ($SU(5)\times U(1)$) supergravity model.

RevTeX 3.0, 16 Pages+4 figures(not included but available as a uuencoded file from gtpark@phyb.snu.ac.kr), SNUTP-94-95