  Starting from the classical action for a spin-zero particle in a
D-dimensional anti-Sitter (AdS) spacetime, we recover the
Breitenlohner-Freedman bound by quantization. For D=4,5,7, and using an Sl(2;K)
spinor notation for K=R,C,H, we find a bi-twistor form of the action for which
the AdS isometry group is linearly realised, although only for zero mass when
D=4,7, in agreement with previous constructions. For zero mass and D=4, the
conformal isometry group is linearly realized. We extend these results to the
superparticle in the maximally supersymmetric "AdS x S" string/M-theory vacua,
showing that quantization yields a 128+128 component supermultiplet. We also
extend them to the null string.
