Upper Limit on Flavor-Changing Neutral-Current Decays of the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>b</mml:mi></mml:math>Quark

We have searched for flavor-changing neutral-current weak decays of the $b$ quark and find ${R}_{B}(b\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}}X)&lt;0.31%$ at 90% confidence level. This limit excludes models with the $b$ quark in a left-handed isosinglet. We also place limits on the ratio of the ${B}^{\ifmmode\pm\else\textpm\fi{}}$ and ${B}^{0}$ semileptonic branching fractions and on ${B}^{0}{\stackrel{-}{B}}^{0}$ mixing.