  We construct the most general N=2 supersymmetric nonlinear sigma-model in
four-dimensional anti-de Sitter (AdS) space in terms of N=1 chiral superfields.
The target space is shown to be a non-compact hyperkahler manifold restricted
to possess a special Killing vector field. A remarkable property of the
sigma-model constructed is that the algebra of OSp(2|4) transformations is
closed off the mass shell.
