We report an upper limit on the fraction of V+A current, fV+A, in top-quark decays, using approximately 700  pb−1 of pp collisions at s√=1.96  TeV acquired by the upgraded Collider Detector at Fermilab. For the decay t→Wb→ℓνb (where ℓ=e or μ), the invariant mass of the charged lepton and the bottom quark jet is sensitive to the polarization of the W boson. We determine fV+A=−0.06±0.25 given a top-quark mass of 175  GeV/c2. We set an upper limit on fV+A of 0.29 at the 95% confidence level, an improvement by a factor of 2 on the previous best direct limit.