The quantity P, where is one of the muon decay parameters and P is the degree of muon polarization in pion decay, has been measured. The value P=1.00030.0006(stat.)0. 0038(syst.) was obtained. This result agrees with previous measurements but is over a factor of 2 more precise. It also agrees with the Standard Model prediction for P and thus leads to restrictions on left-right symmetric models. 2006 The American Physical Society.