In the standard model (SM) of electroweak interactions the positron from the decay of polarized positive muons is mainly longitudinal polarized. However the model also predicts a small transverse polarization componentPT1,which lies in the plane spanned by muon-spin and positron momentum. Interference with additional scalar couplings, as they appear in supersymmetric extensions of the SM, would result in substantial values forPT1as well as in a non zero value of the transverse componentPT2which is perpendicular to the above mentioned plane. A non zero componentPT2,proportional to the imaginary part of a possible scalar coupling, would be the first observation of time reversal violation in a purely leptonic decay. MeasuringPT1,which is proportional to the real part, amounts to a model independent determination of the Fermi coupling constant. The&mgr;PTexperiment at the Paul Scherrer Institute will improve the current experimental limitsPT1=(16±22)⋅10−3,PT2=(7±23)⋅10−3by almost one order of magnitude. Besides a description of the experiment detailed results from the last run in August 1999 will be given. ©2000 American Institute of Physics.