  We search for the purely leptonic decay $B^{\pm} \to \mu^{\pm} \nu_{\mu}$ in
the full BaBar dataset, having an integrated luminosity of approximately 426
fb$^{-1}$. We adopt a fully inclusive approach, where the signal candidate is
identified by the highest momentum lepton in the event and the companion $B$ is
inclusively reconstructed without trying to identify its decay products. We set
a preliminary upper limit on the branching fraction of $\BR(B^{\pm} \to
\mu^{\pm} \nu_{\mu}) < 1.3 \times 10^{-6}$ at the 90% confidence level, using a
Bayesian approach.
