Observation of the baryonic B decay (B)over-bar(0) -> Lambda(+)(c)(Lambda)over-barK(-)

We report the observation of the baryonic B decay (B) over bar (0) -> Lambda(+)(c)Lambda K- with a significance larger than 7 standard deviations based on 471 x 10(6) B (B) over bar pairs collected with the BABAR detector at the PEP-II storage ring at SLAC. We measure the branching fraction for the decay (B) over bar (0) -> Lambda(+)(c)Lambda K- to be (3.8 +/- 0.8(stat) +/- 0.2(sys) +/- 1.0(Lambda c)(+)) x 10(-5). The uncertainties are statistical, systematic, and due to the uncertainty in the Lambda(+)(c) branching fraction. We find that the Lambda K-+(c)- invariant-mass distribution shows an enhancement above 3.5 GeV/c(2).