  We present new limits on resonant tb production in proton-antiproton
collisions at 1.96 TeV, using 1.9 fb^-1 of data recorded with the CDF II
detector at the Fermilab Tevatron. We reconstruct a candidate mass in events
with a lepton, neutrino candidate, and two or three jets, and search for
anomalous tb production as modeled by W'->tb. We set a new limit on a
right-handed W' with standard model-like coupling, excluding any mass below 800
GeV at 95% C.L. The cross-section for any narrow, resonant tb production
between 750 and 950 GeV is found to be less than 0.28 pb at 95% C.L. We also
present an exclusion of the W' coupling strength versus W' mass over the range
300 to 950 GeV.
