  Explicit expressions are derived for the oblique parameters $S$ and $T$ in
certain extensions of the standard model. In particular, we consider
leptoquarks and bileptons, and find phenomenological constraints on their
allowed masses. Leptoquarks suggested by the neutral and charged current
anomalies at HERA can give improved agreement with both $S$ and $T$. If
bileptons are the only new states, the singly-charged one must be heavier than
the directly-established lower limit. Finally, we study SU(15) grand
unification and show that there are regions of parameter space where the theory
is compatible with experimental data.
