AdS(3) black holes and a stringy exclusion principle

The duality relating near-horizon microstates of black holes obtained as or bifolds of a subset of AdS(3) to the states of a conformal field theory is analyzed in detail. The SL(2; R)(L) X SL(2; R)(R) invariant vacuum on AdS(3 ) corresponds to the NS-NS vacuum of the conformal field theory. The effect of the orbifolding is to produce a density matrix, the temperature and ent ropy of which coincide with the black hole. For string theory examples the spectrum of chiral primaries agrees with the spectrum of multi-particle BPS states for particle numbers less than of order the central charge. An uppe r bound on the BPS particle number follows from the upper bound on the U(1) charge of chiral primaries. This is a stringy exclusion principle which ca nnot be seen in perturbation theory about AdS(3).