  We study the semileptonic $B\to{\bf B\bar B'}L\bar L'$ decays with $\bf B\bar
B'$ ($L\bar L'$) representing a baryon (lepton) pair. Using the new
determination of the $B\to{\bf B\bar B'}$ transition form factors, we obtain
${\cal B}(B^-\to p\bar p \mu^-\bar \nu_\mu) =(5.4\pm 2.0)\times 10^{-6}$
agreeing with the current data. Besides, ${\cal B}(B^-\to \Lambda\bar p \nu
\bar \nu)=(3.5\pm 1.0)\times 10^{-8}$ is calculated to be 20 times smaller than
the previous prediction. In particular, we predict ${\cal B}(\bar B^0_s\to
p\bar \Lambda e^- \bar \nu_e,p\bar \Lambda \mu^- \bar \nu_\mu,p\bar \Lambda
\tau^- \bar \nu_\tau) =(2.1\pm 0.6,2.1\pm 0.6,1.7\pm 1.0)\times 10^{-6}$ and
${\cal B}(\bar B^0_s\to \Lambda\bar \Lambda \nu\bar \nu)=(0.8\pm 0.2)\times
10^{-8}$, which can be accessible to the LHCb experiment.
