Searching for mixing of (. mu. /sup +/e/sup -/) and (. mu. /sup -/e/sup +/) with Fermi coupling strength

A search is described for muonium (..mu../sup +/e/sup -/) conversion to antimuonium (..mu../sup -/e/sup +/) with a coupling strength approximately equal to that of the weak interaction. A silica powder target produced thermal muonium in vacuum. A unique sequence of radioactivity following the capture of a ..mu../sup -/ in tungsten was the signal for conversion. No events were seen, implying an upper limit on the effective four-fermion weak-coupling constant for conversion of 0.88G/sub F/ (90% confidence level), corresponding to a branching ratio of less than 2 x 10/sup -5/ with respect to normal muon decay.