  We present a measurement of the fraction f+ of right-handed W bosons produced
in top quark decays, based on a candidate sample of $t\bar{t}$ events in the
lepton+jets decay mode. These data correspond to an integrated luminosity of
230pb^-1, collected by the DO detector at the Fermilab Tevatron $p\bar{p}$
Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the
kinematics of the $t\bar{t}$ and decay products, which allows for the
measurement of the leptonic decay angle $\theta^*$ for each event. By comparing
the $\cos\theta^*$ distribution from the data with those for the expected
background and signal for various values of f+, we find
f+=0.00+-0.13(stat)+-0.07(syst). This measurement is consistent with the
standard model prediction of f+=3.6x10^-4.
