  It is shown that the parity--violating deep--inelastic scatterings of
unpolarized charged leptons on polarized protons, $\ell^{\mp} + \vec P\to
\stackrel{\scriptscriptstyle(-)}{\nu_{\ell}} + X$, could provide a sensitive
test for the behavior and magnitude of the polarized strange--quark density in
a proton. Below charm threshold these processes are also helpful to uniquely
determine the magnitude of individual polarized parton distributions.
