  If the recently discovered anomalous events at HERA are due to a scalar
leptoquark, then it is very likely to have weak isospin $I = 1/2$. In that
case, present precision measurements of the oblique radiative parameters $S$
and $T$ provide strong constraints on the mass of the other component of this
doublet. If the standard model is extended to include such a doublet, a
slightly better fit may in fact be obtained. However, in specific proposed
models where this doublet comes from a larger symmetry, there are often
additional large and positive contributions to $S$ from exotic heavy fermions
which far exceed the present experimental limit. A way to improve the Tevatron
exploration of leptoquarks is proposed.
