We report the results of a search for the rare baryonic decays $B^0\to p\overline{p},$ $\Lambda \overline{\Lambda },$ and $B^{+}\to p\overline{\Lambda }.$ The analysis is based on a data set of $31.7\times {10}^{6}B\overline{B}$ events collected by the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. No statistically significant signals are found, and we set branching fraction upper limits $\mathcal{B}{(B}^0\to p\overline{p})<\mathrm{}1.2\mathrm{}\times {10}^{-6},$ $\mathcal{B}{(B}^0\to \Lambda \overline{\Lambda })<\mathrm{}1.0\mathrm{}\times {10}^{-6},$ and $\mathcal{B}{(B}^{+}\to p\overline{\Lambda })<\mathrm{}2.2\mathrm{}\times {10}^{-6}$ at the 90% confidence level.

• Received 19 March 2002

DOI:https://doi.org/10.1103/PhysRevD.65.091103