  We report an upper limit on the fraction of V+A current, $f_{V+A}$, in top
quark decays, using approximately 700 pb$^{-1}$ of $p\bar{p}$ collisions at
$\sqrt{s}$=1.96 TeV acquired by the upgraded Collider Detector at Fermilab. For
the decay $t \to Wb \to \ell \nu b$ (where $\ell=e$ or $\mu$), the invariant
mass of the charged lepton and the bottom quark jet is sensitive to the
polarization of the $W$ boson. We determine $f_{V+A} = -0.06 \pm 0.25$ given a
top quark mass of 175 GeV/$c^{2}$. We set an upper limit on $f_{V+A}$ of 0.29
at the 95% confidence level, which represents an improvement by a factor of two
on the previous best direct limit.
