A study of the reaction e+e− → μ+μ−+ anything at a mean centre of mass energy of ≈35 GeV has been carried out to investigate the possible flavour-changing neutral current decay of the b quark. An upper limit of 0.7% at 95% CL has been placed on the branching ratio for this process. This limit conflicts with the predictions of a five-quark model (without top) which assigns the b quark to left- and right-handed singlets.